The challenge here is to have the British Empire gain control of (and hold until the breakup of the empire) an Eastern European region. For the purposes of the discussion, an “Eastern European region” is an area east of present-day Germany with majority Slavic population* or a region in the Balkans (regions of ex-Yugoslavia, present-day Albania, Hungary, Bulgaria, Romania, etc.) or the Baltic.
* if the Slavic majority is reduced to a minority via genocide or a flood of British settlers, it still fulfills the requirements.