Its not illegal per se to cross the broken lines however check out rule 106 of the highway code ok so mine is the '04 edition and could be considered out of date.
Nevertheless:
106: A broken white line. This marks the centre of the road. When the line lengthens and the gaps shorten, it means there is a hazard ahead. Do not cross it unless you can see the road is clear well ahead and wish to
overtake or turn off.
Also imagine this if you will
Officer: I'm going to caution you now. You do not have to say anything, but anything you do say may be given in evidence in a court of law. Why were you weaving across the road back there?
Driver: I haven't done anything wrong, I was using the racing line...
Officer (interrupts): does this look like a race track to you Jenson? do you know how many people have been killed on this section of road? etc etc
so if you do decide to use the "line" then keep your eyes
cause if maybe the two guys in the blue 3.0 V6 Omega behind you are
.
I'm not trying to tell any one how to driver their car. I'm just presenting the facts with a bit of worst case scenario. It's up to you all to decide for yourselfs
.