For BestBy dates, that's usually up to whoever does your testing and authorizing. I don't know how it all works in the UK and who your health authority is, but it is up to them to set it all out.
Here in the US, we send a recipe & sample to the lab, they review everything, do pH testing, whatever is required, and then will issue a Process Authority letter which says how the sauce can be made and packaged based on the ingredients. I have a fresh refrigerated salsa that has a pull date of 30 days from when it was made. The bottled hot sauces don't have a pull date. They pretty much won't go bad. BUT!!! I have had bottles of sauce from other companies in the fridge for (3?...4?...5???) years and they have DEFINITLY gone bad. They were 3 years out of the BestBy date, so there you go~
If you are looking at doing a lot of small run batches, you will likely have to send each one out for testing and approval since each recipe is different.
Good Luck and Have Fun!!!
Again, I'm not familiar with protocol in UK. Your first step should be contacting your local health authority and find out what they want. Here in the US, regulations can vary drastically from state to state, and depending on where the processor is selling.