Worst I've ever done was Guildford to Luton. The night shift 19:00 - 07:00 was the worst, you'd hit the traffic on the M1 straight away and then crawl all the way back to the A3 junction... dreadful, would take me anything up to 2 hours (by which time I'd fallen asleep about 5 times in the car - stupid really). That northwest section is the worst bit, dread to think what it'll be like for the next 3 years with all that carriageway widening!
You should be OK on that south west bit of the M25, at least it moves; it'll slow down after the A3 junction though, all the way to LHR - about 7 miles I think.
Never again though, I'd move as soon as I could (though then we get onto house prices, which is a whole topic in itself!)