I stick to 70 at a max, seeing generally no need to go any faster. 70mph is fast enough. I might use a bit more to overtake something (allowing for the ~10% over read that I know exists in my car), but I'll stick to what my car says is 70mph, and that's fast enough for me. More often than not, I tend to be a bit further down because it isn't even safe to reach 70mph.
However, I do cautiously support an increase to 80mph, but it should be applied to certain roads only. Some motorways are fine with a 70mph speed limit, but in some cases there is justification for an increase. The road I think of first and foremost is the A74(M); it's a very hilly motorway, and as such it is very easy to creep up to 80mph without noticing it!
The problem with limits in general is that some morons see them as targets. On a busy motorway, 70 is often far too fast (I have first hand experience of the M8, where you're lucky to do 7mph never mind 70!). Increasing the limits in general is only going to encourage people to go faster than they otherwise would, and that's not a good thing on every road. IMO, the motorway limit should be left at 70mph, but increased to 80mph in specific cases where it would make sense. I would suggest that it's only applied to motorways with three or more lanes, for example. I've cited the A74(M), but I'm sure that there are other cases in England and Wales. Definitely, the majority of motorways I've been on are busy and fast enough at 70mph.
And, if you were to increase it, an 80mph speed limit should be very strictly enforced IMO.
Interestingly, there's talk about Scotland gaining full control of speed limits. The papers have been opposed to this (and printed scaremongering headlines such as "Holyrood gains power to cut speed limits"), but it will be interesting to see if any limits actually increase as a result of such a proposal.