There is Cakes of Good Taste in Watford http://www.cakesofgoodtaste.co.uk/ and there is also Faircake in Greenwich https://www.faircake.co.uk/.
I have not attended any classes at Cakes of Good Taste, but have met the owner who was lovely. I have attended a few classes in Greenwich awhile back and they were great, they also have great tutors come in as well. Take a look at the websites and see what you think.
Sweet Fusion Cakes