pay a lot of money to the British Embassy in Warsaw
Wherever you get married you're going to need to do this and do it in that country's embassy. Well, you can do all this remotely of course, but they'll still need to be done.
I had a church wedding, I don't recommend it, we had to jump thtough all kinds of mad hoops, even attending marriage courses, where we learned about the evils of contraception! Lol.
Doing it in a registry office here though it pretty easy, you need that UK embassy document...how much is it? My Irish one was I think 60euros, but I think that's the only state document I needed, I needed a whole ton of church paperwork but that doesn't apply to you, luckily.
If I was in your boat, I'd just do it here, Poland's pretty cheap to get married in and weddings pretty much pay for themselves (so long as you ask enough people to come that is) and importantly, you don't want to pi$$ off the wicked tesciowa already. Remember, she's your free babysitter when that day comes ;)
You could looks at options in Czech & Slovakia, that way most invited guests could still make it, but y'know, they have similar legal systems so the requirements will be similar, plus your fiance will have to submit and get here paparwork in order too coz she won't be a citizen of those places. I think you'd be making it far handier on yourselves to do it here. Trust me, you'll already be stressed and have arguments about organising it so my advice is to try and keep those to a minimum.