Does the U.K. military mix up the men that they recruit from different regions into same group that fight together?

Question

like do they mix up Scottish, irish and English, welsh together or do they kind of try to put English with English, Scottish with Scottish, etc?

how this was similar or not during WW1 and 2? and why?

Answer 1

Yes they do

they had a thing called "pals battallions" in WW1, but stopped when it turned out that hundreds of simultaneous casualties from the same village was very detrimental to morale.

Also, the Anglos did (do?) not want entire army-units speaking Gaelic or Welsh

Answer 2

I would imagine many units do but there are a few traditional units like the Gurkhas, several Scottish units and Irish units that are drawn from their home region or nation in the case of the Gurkhas.