From what you've written, it seems evident that a court would be convinced that you punched the other guy and broke his nose. Who started the fight is then largely irrelevant, as to successfully defend your actions as 'self defence' would require you to show that you were using the absolute minimum force necessary to ward off an attack. (In many cases the only valid 'self defence' is to turn and run. If you genuinely can't do that because, say, an aggressor is in your path then the only 'self defence' open to you is to simply push him out of the way and then run. It's only if that option is also blocked that you have the right to strike a blow and, once again, you must use only the minimum force necessary. I doubt that you could convince a court that you only acted because you had no other alternative).
You should obviously ask your solicitor (who will have more detailed information than we have) about the best way forwards but, as I see it, pleading 'not guilty' would be counter-productive, in that you'd almost certainly be found guilty anyway and simply end up with a more severe sentence.
There seems to be nothing in what you've written to suggest that your case could end up before the Crown court (unless, of course, you exercise your right to demand trial by jury), so it's the sentencing guidelines for magistrates which you need to look at:
https://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG-April-2017-FINAL-2.pdf
(See pages 177 to180)
You'll see that the magistrates (or District Judge, as appropriate) will have to decide whether the offence falls into Category 1, 2 or 3. I can't see anything in your post which suggests that 'culpability' could fall into the 'higher' section and, as a broken nose is a relatively minor injury, it seems unlikely that 'harm' would be regarded as 'higher' either. If the magistrates agree with me then it's a Category 3 offence, for which you CAN'T be sent to prison.
Even if the offence was seen as within Category 2 (where the 'starting point' sentence is 6 months imprisonment) I suspect that the magistrates could find reasons to push the sentencing towards the lower end of the available range (but possibly only if you've pleaded guilty and displayed remorse), so once again a custodial sentence would seem to be unlikely. Even if such a sentence was actually passed there would be a fairly good chance of it being suspended.
My best bet is that you'll end up with a community order.