They fought at the siege of Terra and went nuts after their primarch died, the curse haunts them forever more.

Everyone wants it to be a military disaster, but that sort of thing tends to leave more evidence than we have. I've always felt some kind of boring bureaucratic end is more likely.

They were corrupted by Chaos and cast away into the warp.

Can we knight this OP for actually providing context??

stationed in Britain

Well there's our answer. Scotland!

The Scots happened to it!

Yes ! Context ! The 🐐

They went through an interdimensional portal in order to help a bunch of Celts stop an invasion of light eating alien monsters.

They're in Alera with the Pokemon, Zerg and Khajiit!

For more information on this, watch the 2011 documentary The Eagle.

They followed one of the lost Primarchs.