I know it may sound crude, but have you tried Pubs, I know that may not be your first choice but normally they do tend to have decent, Local authority approved kitchens, and if you can fit in whilst they are not using theirs or you may be lucky enough to find one who don't use their kitchens and are willing to rent it out 100% for you.
Pubs are having a hard time at the moment so it'll be easier for find a landlord that would say "Yes" and at a much cheaper price that a commercial kitchen, which you can go for should your venture need larger premises.
Get some Cards printed with what you do and give one to the landlords next time you go out for a drink