The legality of how much force can be used is pretty straight forward. In a nutshell, the force has to be reasonable in the circumstances.
It is up to the individual concerned to prove he was acting reasonably.
Reasonable, is a term used widely. For example both bouncers and the police do not have any special powers regarding the use of force (powers to search etc are different) but the actual use of applied force is the same for the Chief Constable of the Met as it is for Mrs Miggins. And one must remember, the Police kill people!!!!!
In my humble legal opinion, a choke hold and a drag would be reasonable in these circumstances. You brother hitting his head is not reasonable, but sounds like an accident. Although the result will constitute an assault (the apprehension of unlawful personal violence) if the case goes any further, a criminal "mens rea" or "guilty knowledge" of an assault will have to be proved. Yes, there is a crime as reckless assault, in this case the bouncers should have had the foresight in realising that if somebody is thrown to the ground it may cause more injury than is necessary. That is a case for the crown to prove, not a case for the defence to justify, therefore making it harder.
One does not have to fight to be seen as a threat. The situation as a whole, body language, the amount of possible aggressors near by, proximity to weapons (ie glass bottles etc) are all what are known as impact factors in deciding whether reasonable force was used.
It is solely on the say of the bouncer testimony. "I felt threatened, there were four fully grown men fighting, there were bottles nearby, I felt for my safety and those of other colleagues and the public etc etc etc".
Ask yourself this. Can such testimony be proved wrong beyond all reasonable doubt??? If not he won't have a gnats chance in hell in winning any case.