My sincere advice is do NOT proceed with this. Here's why:

1) As a GCH you did not actually pay anything. Yes, there is normally a charge to reserve this seat, but BA are not legally required to compensate you for something that they have given you for free. In any event, even if you did pay for the seat, you still got the extra legroom you paid for; the fact that the seat didn't recline does not give rise to a compensation claim for value paid for extra legroom. Technically speaking, you may have a legal claim for breach of contract over the broken seat (and not being given a suitable replacement), but good luck quantifying your loss and in my experience it is a complete waste of time and money bringing a legal claim for something which will never be of significant value (and you will never be able to recover your costs in the small claims court).

2) I have no proof otherwise, but if you do decide to sue BA do not be surprised if your profile with the airline is flagged for negative reasons (meaning that you might find yourself not getting any upgrades, etc). Companies tend to treat customers that sue them differently from those that don't!

I know you are angry with the way you have been treated, but take some time to step back from this and consider whether it's worth your time and effort taking this forward. Sometimes you just can't beat the phone company......