[Rainbow Six]

TGirl Kelley is 100% correct.

Historically the British Army Regimental system was much more of a thing. These days, with recruitment being what it is, new recruits can express a preference for a particular Regiment but can in theory end up anywhere that they are needed (back in the day when the Army was bigger probably more likely that you would end up in your 'local' Regiment).

So to answer your follow up question, you can't choose per se, just express a preference. And yes, it is my understanding that some Regiments are more popular than others so essentially it's a sort of interview process (having links to the Regiment (e.g. father / grandfather served in it) will help, as will being in the upper echelon of your recruit class.

I don't have any sources for this but from memory the Welsh Guards historically haven't had much problems filling their ranks with Welshmen whereas the Scots Guards did have a problem attracting Scots. I don't know about the Irish Guards.

But if you want to (for example) have an Englishman in the Welsh Guards there's no reason why you can't.