Walls of text apologies in advance, I'll hopefully cover your initial questions and some obvious follow-ups.
I'm UK also and have run small systems for plant breeding so I can give you some personal recommendations. Lots of different ways to do this and it depends on what you want to achieve. From my years of growing in different systems I'll just give you a
very opinionated view!
My answers to your questions followed by some recommendations.
Unless you are going with some extreme mix, which I wouldn't suggest in small systems, you can basically ignore this if you pick a balanced product. You are running a couple of small systems, you aren't going to be growing monster plants and trying to maximum yield. Beware of product differentiation which aims to make you buy multiple products. Just because a flower appears doesn't mean you need a different product. All fertiliser related products have gone up in price over the last couple of years, nutrients are no exception, beware of nutrient hype though!
Your plants will grow, you might get lucky and they might be happy for their lifetime. Some of those generic nutrients focus purely on NPK and don't always have all the other micro-nutrients your plant will need to be happy. If a specific product does have extra micros it probably has them in ratios that are aimed at soil supplementation rather than total absence of many micros in plain water. Hydroponic nutrients are basically designed on the basis that they need to supplement water or inert media.
As mentioned above, most hydroponic nutrients will have a balanced profile across the engine room needs (NPK) and complementary, but essential, micronutrients. There are cheaper hydroponic nutrients and more expensive hydroponic nutrients, you can choose which to buy. Generally the bigger the amount you buy the cheaper the unit price becomes. Liquid nutrients are convenient and can be purchased in small bottles to give hydroponic growing a try, they will look relatively expensive compared to bulk purchases. If you are just dabbling to see if you like it you don't want years worth of nutrients to feed a forest. A little goes a long way. Powdered nutrients are cheaper than liquid, they are just as good but you need more work to mix them and they tend to come in bigger packs, so you'll likely be buying too much for your needs.
I don't know where you plan to have these systems? In the house, in a greenhouse or outdoors? Indoors, just look out for any insect larvae in your water if bothered about that. Indoors will need lighting but I'll assume you know that already as you are growing things. Ideally you are going to want a way to check the pH of your source water but more specifically your final mix. Cheap pH test strips if you want, cheap pH electronic meter. If you get into this big time I'd recommend a higher quality meter but you don't need that right now. Some specific nutrients can help you on the pH front also but you still need to test when starting out so you have some idea what you're working with. When you add nutrients you are typically lowering the pH of the source when using tap water in the UK. Rain water will be a lower pH to start with and I've not used it for hydro so you'll have to see what you have. You'll either want to buy some pH down or pH up when you know what the situation is, small bottles last ages. Use alternative adjusters if you like, I prefer just using a commercial product.
Take a trip around social media and you'll mainly see products which are prohibitively expensive when buying in the UK versus the US. Dyno-gro, Fox Farms and similar are great I'm sure but can be pricey over here. Masterblend tomato formula is good, it's a powder form, but unless you find a deal it's also expensive and in big packets. Aerogarden, 1 part liquid, hydroponic nutrients are good enough for small kratky but can also be overpriced in the UK. I've personally used Masterblend and Aerogarden in the past.
You'll find you can find 1, 2 or 3 part products. The only difference is the more parts you have the more time it takes you to mix and the more ability you have to tweak the mix. 3 part often means you need to buy more, e.g. 3 * 1L rather than 1 * 1L, so initial outlay could be higher.
So eventually, I hear you say, what do you use?
Things that have worked for me:
- General Hydroponics Europe/Terra Aquatica Flora,
3 part, 3 * 1L around £34
- Advanced Nutrients pH Perfect (Micro, Grow, Bloom),
3 part. 3 * 1L around £38
- Advanced Nutrients pH Perfect Sensi Coco, 2 part Bloom,
2*1L around £24 or you can get half as much for £19
- Canna Coco A+B, 2 part,
2 * 1L around £17
For reference I use this for
pH up and pH down, I only need to use pH up if I filter my source very hard tap water and use
this CalMag when I find plants need more.
From the nutrients above:
- The two 'coco' branded products work fine in non-coco systems, e.g. water
- The two Advanced Nutrients products are more tolerant to wide pH ranges. I've used them just added to tap water and no pH adjustment, my source water is around pH 7.0. Ignoring the marketing on their pH Perfect technology, plants generally do fine even without checking pH. Sometimes you'll find a plant complains but the exception rather than the rule in my experience. The
best way to do things is to check and monitor pH, if you want lazy then this product range gives you a chance mostly.
- The Advanced Nutrients Sensi Coco product comes in 2 part Grow and a 2 part Bloom products. They are very similar in content, you can just run with the 2 part Bloom and be done with it if you go that way.
- The GHE/TA and Canna products need more pH adjustment, that means checking the pH of your final mix is in an appropriate range, 5.8-6.2. Fwiw I aim for the lower end of that range. If you ever go hydro big time then GHE/TA Flora 3 part is hard to beat for quality and tweaking, it's my preference for big systems.
- The Canna Coco product is much cheaper than the others, so might be your best way to play. I've only used it in a limited number of kratky but used it for coco last year. I personally won't be using it again as I found it not as good or versatile as my go-to Advanced Nutrients products. That said, I grow lots of different species and get fussy. It's a solid product for your needs if you want to pH adjust, same pH range as mentioned above.
-
My personal, default, choice for small kratky systems is the Advanced Nutrients pH Perfect 3 part. Minimal hassle, handles pH fluctuations OK and keeps me lazy.
Bottom line, the two cheapest routes for you:
- Hydro nutrient route: Canna Coco A+B, pH test strips/cheap meter and then buy some pH up or down if rainwater + Canna mix doesn't hit a desirable pH range
- Generic nutrient route: Chilli Focus,
1L for £12, 5L for £20. Not a hydro nutrient but I've used before as it has lots of micros in it. You just need to monitor pH closely and any deficiencies you spot on individual plants. Get some ph test strips/cheap meter, might need pH up or down. It's also fine as a good balanced feed for soil based plants.
In case you wondered, I have no affiliation or relationship with any of those brands or products mentioned above, just what I've spent my own cash on over the years.
Example of small system from a few years ago, 240mL jar
I've made a simple Kratky set up got a couple of spare plant's I want to try it on, the only thing holding me back is nutrition side !
Thanks
yes the sickness grows 
This hydroponic experiment with "spare" plant's has really become interesting 
