If your fiancee already has a job in the UK, that makes things much easier. If you get married in Poland that means making an application for some sort of a Polish Resident Permit before the British Embassy in Warsaw will even agree to touch your passport.

If you qualify for a registration of marriage in the UK under the new laws that will be the best option.

The third option is to apply from your Home country but willing to endure all the crap that the Immigration Authorities give African applicants from their countries.