The last bit of that route turned out to be a London Cycle Network special - charming residential streets with double-parking and just about room for one car up the middle. Oh, and sleeping policeman at monotonously regular intervals. It also took me under the North Circular along what looked just like the start of a motorway - OK mid-morning, but I'm not sure I'd fancy it closer to the rush hour.
To come home I concocted a route which eventually debouched me onto the A404 somewhere in Kensal Green, and I just followed it all the way home. The North Circular junction is sensible, and most of the busiest parts of the road either have bus lanes or bike lanes with oodles of room around them. Coming away from London the gradient is absolutely relentless - my GPS track does show some downhills, but they're heavily outweighed by the ups. I do wish I'd taken lights - almost all of the cars already had their lights on come 7pm. It is only a week after midsummer...
I reckon the A404 down into London is a real goer. I clocked 2 hours dead to Euston. Simplifying the route, not losing my way and taking a lighter bike could probably eat 15 minutes out of that time. Home again is a bit of a different story - my time was 2.5 hours, and it was one of the most tiring rides I've done. It would be easier on my best bike than my second-best, but I'm wondering whether there's more mileage in riding to Harrow (perhaps along the canal, as suggested upthread) and then getting the train the rest of the way.