Just to be clear I totally agree with others (ref solicitors) who have posted since you posted these questions. That you are unlikely to need to use one at this stage, and that if you do use a solicitor it will come at significant cost to you in legal fees - as you have found out by seeking some quotes.
It's a sensible thing to do to get quotes of course.
Ref your specific questions:
I don't know anything about the legal firms mentioned here or other firms, beyond what I read on there - as I've never been accused of fare evasion / ticketing irregularity myself, so I can't make a recommendation based on experience.
However, I did use the search facility on the forum and it brought up a small number of mentions of those firms you have called - so you could do that search and have a read.
I also think it's worth mentioning two other firms that get mentioned on here by people reasonably regularly who have used them, and I think you would find more mentions. You could also search the forum for those examples if you wanted to see what people say.
These are:
Manak Solicitors
Skilled legal defence for fare evasion cases. Rely on our experienced team to protect your rights, challenge penalties, and guide you through the process.
manaksolicitors.co.uk
Makwanas
Facing a fare evasion prosecution? Our UK fare evasion solicitors help you settle out of court and avoid a criminal record. Speak to us today, free review.
www.makwanas.co.uk
But I would set this against the great many cases on this forum which are resolved with good advice form people here and no solicitor involvement at all. Sometimes this includes quite complicated cases.