Nailit:
The three category system referred to in my posts above (and in my Sentencing Council link) didn't exist when you came before the courts, so it can't be applied to your situation.
AM:
As has been said several times already, it's really your solicitor's advice which you ought to be taking. However perhaps you ought to also think how the magistrates might see things.
Although the law is meant to work on the basis of 'innocent until proven guilty' there can come a point when people's minds start to work the other way round.
If you appear before the court and don't dispute that (a) you swung a punch and (b) that punch broke someone's nose, the magistrates are likely to take the view that you're clearly guilty of ABH unless you can show a damned good reason why they should find otherwise. (i.e.the point has been reached in their minds where it's effectively up to you to prove that you're innocent, rather than up to the prosecution to prove you guilty). Given that you've indicated that the only witnesses are likely to be hostile towards you, that could turn out to be extremely difficult.
Hopefully I've explained my reasoning why it MIGHT be best to plead 'guilty' but I'll repeat once more that you should listen to your solicitor.