Unfortunately this industry is run on the most part by people who have absolutely no idea how to run a business properly.
Self-employed FI's have long been a dodge to try and keep costs down. It's no wonder HMRC are looking at this closely.
If you teach part time on a non-regular basis, then fine you should be self-employed, but for most FI's they should be employed and it is only because it is easier for clubs/schools to dictate self employment status on thier staff. It gives more latitude for acting badly aswell.
This is being looked at very closely at the moment by numerous agencies and if it falls against the clubs, then there may well be a fair few going under, since they would be liable for all the employer's NI contributions that they haven't paid.........