I guess a flying school / club should be no different from any other kind of business though.
If an instructor works EXCLUSIVELY for one school / club on a 'self employed basis', then a 'master and servant' relationship will exist. In this situation, HMRC will try to argue that that the relationship is in fact 'disguised employment'.
On the other hand, if an instructor works on a self employed basis for a number of people, he / she is genuinely self employed and HMRC can then have absolutely no issue with the arrangement.