Except one lane is often dominated by a type of vehicle that legally gets limited to a lower speed than the other vehicles

And? Makes no difference. It's this attitude (that's the slow lane) that wastes so much motorway capacity. If you were on a single carriageway road overtaking, you wouldn't stay on the wrong side of the road because the correct side 'is often dominated by a type of vehicle that legally gets limited to a lower speed than the other vehicles', you would overtake, and return back to the left. It really isn't any different on the motorway.

Except if you drive at 70mph which is recommended for traffic flow

Slight correction, 70mph is the limit, not a recommendation. It's a limit not a target. In good conditions 70mph is fine, but it's not really correct to call it a 'recommendation'.

then you'll be constantly changing lanes in and out

If you genuinely are 'constantly' changing lanes, then I think even the highway code acknowledges and recommends not moving back for a couple of seconds. But 20 seconds isn't 'constantly'. 20 seconds is time for others to overtake.

There's just no need.

Yes there is, it's called consideration for other road users, and doing so increases road capacity.

If changing lanes is dangerous for you, you are doing it wrong. If you find driving in the driving lane and then changing lanes to overtake dangerous, you always have the option of not overtaking - ie driving at 56mph so you can remain in the traffic flow with less overtaking.

I find changing lanes to overtake helps me keep my concentration up rather that risk slipping into 'autopilot', which I'd argue is dangerous.

If you get ever get a chance, drive on the Autobahn in Germany. Much of it is only 2 lanes, but due to generally exceptional lane discipline, they seem to get better capacity out of their 2 lanes than we do our 3 :/ They do exactly what you are against. Immediately after passing a slower vehicle they move back to the right (in their case) so other traffic can pass, even if only for a few seconds. In turn approaching traffic expects traffic people on the right to move out if they are catching a slower vehicle so you don't end up 'trapped' in the right lane. Despite often having vastly different speeds between the two lanes (much of it has no speed limit) It generally works brilliantly. It's so depressing having driven across Germany to come through the tunnel and almost immediately encounter somebody sat in the second lane, because they don't want to 'constantly' change lanes.

Tl;dr: Keep left unless overtaking, it's not that complicated!