The solicitors will want something in advance. How much is the something? Or are they financing the expenses of launching a trial themselves, and billing you later?
If you lose, you pay all your own lawyers' costs plus the lawyers' costs of the other side. That's the normal rule. If you win, the likelihood is that you will not recover all your costs from the other side.
I cannot comprehend, if you have told solicitors and counsel what you have told us, and we have the whole story, how any ethical lawyer would advise that you can win this. There are professional lawyers on here, in this thread, who take a dim view of both the sums you have been quoted and your prospects .
You have a solicitor and , apparently, counsel. Obvious, but if you have them , why are you asking complete strangers ?