I wanted to check this, so I looked it up.

168 Being overtaken. If a driver is trying to overtake you, maintain a steady course and speed, slowing down if necessary to let the vehicle pass. Never obstruct drivers who wish to pass. Speeding up or driving unpredictably while someone is overtaking you is dangerous. Drop back to maintain a two-second gap if someone overtakes and pulls into the gap in front of you.

The wording of that is ambiguous enough that I imagine a) it's unenforceable and b) your average lorry driver could (would?) say that it's not necessary to slow down.

It does go on to say that you mustn't impede the flow of traffic by overtaking, but since presumably everyone is still actually moving at a reasonable speed relative to the limit they're not strictly speaking doing that either.

Don't get me wrong, it winds me right up when I'm stuck behind it, but so far as the highway code goes they're on the light side of grey.