SteveH opined the following...
That's the offence of 'without due care' or 'dangerous driving' - not
undertaking.

Given it would be your word against plod... I think you'd be booked
faster than you could say "Undertaking is not illegal"!

In addition. Were you to be involved in an accident, you might have
trouble with your insurers as they point to the HC and suggest that you
try following its "guidelines" next time.

And do try to relax a little. Ride your bike more. The time sat in your
car is obviously not doing you any good!

Jon