The UK Highway Code is fairly clear about overtaking on the wrong side. It's not against the law as such, but courts use it as a basis for determining whether you've been driving in a safe way. At the same time if someone is hogging the outside lanes, rather than using them just for overtaking, they are in breach of the Highway Code as well. Even so, in any accident arising from this manoeuvre, the person overtaking on the wrong side would most likely be seen as primarily responsible:

Dual carriageways

Section number 116 On a two-lane dual carriageway you should stay in the left-hand lane. Use the right-hand lane for overtaking or turning right. If you use it for overtaking move back to the left-hand lane when it is safe to do so.