The perceived wisdom (!) regarding filtering is that it is a very grey area, and ideally your overtaking speed should be approx 20mph above that of the traffic being passed, so if they are stationary, you should be going no faster than 20mph. Should you have an accident, even if it isn't 'your' fault, ie car suddenly turns right on you, your insurance provider is likely to go for a 50/50 settlement, so you you'll probably have a fight on your hands getting them to back you.
For moving traffic, I'd stick to lanes 2/3, unless the inside lane is empty, which it often is.
Once things start to get moving at a good lick, I'd say you're no longer filtering, but over/undertaking
This can of course be done at any speed you like, as long as the gap is big enough, and there is no sign of Mr Plod . . .
The best piece of advice given to me was -
If you have to think "Can I make it", you probably shouldn't try to
