[lee martell]

Is this ‘a thing’ in the UK?

[Dr. Sivana]

Is this ‘a thing’ in the UK?

It sure was in Wodehouse. Several of the women Bertie had been engaged to were his cousins.

[Lee N. Field]

Is this ‘a thing’ in the UK?

It's a thing in a lot of the places many current UK residents come from.

[EnderWiggin1970]

Probably more of an issue with Muslim immigrants, but yes, it was commonplace in the past. Charles Darwin married his first cousin for example. The evolutionary thinking of the time was that “superior” folks should keep breeding together in tight circles to keep their gene pool pure and strong (think of royal intermarriage for example). Ironically it was the worst possible approach to this topic.