"changing lanes on a British Motorway you only need to indicate when pulling out not when pulling in."

I've heard multiple people say this in the past, including driving instructors(!!!!!) and it's just not true.

Highway Code article 133 clearly says:

"Lane discipline 133 If you need to change lane, first use your mirrors and if necessary take a quick sideways glance to make sure you will not force another road user to change course or speed. When it is safe to do so, signal to indicate your intentions to other road users and when clear, move over".

You always have to indicate when changing lanes. There is no distinction made between pulling in or out by the highway code, I honestly think people made it up in their heads and they keep to it - maybe because you don't need to indicate back when overtaking on a single lane road, but that doesn't apply on multi-lane carriageways. However the point seems to be mostly academic as in my experience most people don't bother indicating at all on a motorway, whether pulling in or out.

"since undertaking (except in slow moving traffic) is also ilegal."

Highway code doesn't mention anything about slow moving traffic, just "similar speeds" - so it's perfectly legal to undertake a vehicle going 68mph when you're going 70mph, if the traffic is heavy:

"Rule 268 Do not overtake on the left or move to a lane on your left to overtake. In congested conditions, where adjacent lanes of traffic are moving at similar speeds, traffic in left-hand lanes may sometimes be moving faster than traffic to the right. In these conditions you may keep up with the traffic in your lane even if this means passing traffic in the lane to your right. Do not weave in and out of lanes to overtake."