To clarify, you're pleading guilty - okay. So none of this really makes a difference; you're not trying to make a defence out of it, you accept the offences, and you will be convicted of the single offence and have the remainder of them taken into consideration.
There is so little movement on the level of fine a solicitor would be a waste of money at this point. A fine is linked to your income and there's not a huge deal that can be done about that, bar a small adjustment up or down - but the £1,139 in fares avoided is not something you will be able to dispute given you accept them, nor the surcharge, or prosecution costs which are standard and seem reasonable.