Seems it's a grey area
Involuntary undertaking (where the right lane slows down but you don't) is legal. Deliberately changing lanes to undertake is considered dangerous.
(localboy purveyor of pisspoor puns, Thu 2 May 2013, 10:36, 2 replies, latest was 12 years ago)

The trick is in timing it properly

( Kroney, Thu 2 May 2013, 10:37, Reply)

Like I did on the M5 the other week
in lane 1, 40 Audis nose to tail in lane 3 as usual, hoggers in 2, junction comes up, brake lights all over, 2&3 stop dead, me and the wagons happily carry on.
Some of the twats in the fast lane didn't catch us up for 50 miles or so.
(localboy purveyor of pisspoor puns, Thu 2 May 2013, 10:41, Reply)

I think it is perfectly acceptable for me to undertake at 70mph when I am already in the correct lane.

( tangledupinblue hairy badge with moving eyes, Thu 2 May 2013, 10:40, Reply)