Beattitude is an all female gym on Garratt Lane. It is entirely class based but with all the kit, small and very friendly. They ran a teen class for my daughter & friends who started when she was 14 and she still attends at 18 and loves it..
There’s a complete age range from 15 to 60+. Classes are usually about 10 people. strength, cardio, barre, yoga etc.
Take a look at the website https://beattitude.club/
Siobhán the owner is lovely. Lots of intro offers so she could try it.
Interestingly just about to get my younger daughter (also now 14) started there so hoping to persuade Siobhán to run another teen class in September.