Ignore wiltsbadger. That is just nonsense. The definition of self defence is now in a statute, which reflects what earlier cases have established as the law [ Criminal Justice and Immigration Act 2008, section 76 and in particular section 76(7) ]. You have a perfect right to defend a friend. You are not expected to weigh with precision what force is strictly necessary. There is no duty to retreat. If what you do seems reasonable and necessary to you at the time, when acting instinctively and honestly in defence, that is very strong evidence to show that it was self-defence.
Your barrister is concerned that the need for any self-defence had gone and you had gone from defence to attack, to revenge. That is as bad as it gets from the defence point of view, but it is very far from a serious difficulty. It is not as though all the violence had clearly ended and you then chased one party two hundred metres down the road and hit him with an iron bar ! What you did could fall under 'instinctively and honestly' (above)