I don't know about that when I first trained MMA back in 2006/7 there was one gym around, now a few have come and go but there's still a fair few to choose in comparison. The event times are a big issue here, with big boxing events like Joshua they are in the UK and shown live in bars, MMA is primarily shown in the US so hasn't become something casuals will just encounter at their local and learn to accept, therefore not as many people seek to train this sport.
Wrestling is also the base for successful MMA and the UK has close to zero wrestling schools, we don't train it through school and simply never will, try telling a UK kid to lycra up and wrestle his pal and he'd run a mile hahaha, but because of this we learn wrestling through MMA, from a coach that learnt it through MMA and so on, so it's hard to improve that vital part of the game.