It really does depend on the route you take. I don't take the shortest route as that involves lots more junctions, more lights and more conflict; my route algorithm is mostly about minimising stress.

7 mile commute in London and analysis of my GPS tracklogs show that, on average, I am stopped at 5 out of the 23 traffic lights there are on my commute with only 3-4 minutes of stopped time on a 30 minute commute.

It's helped by long sections of nice wide road with few junctions where good progress can be made. The best example of this is Battersea Bridge to Lambeth Bridge (North side of the river) so going along Chelsea Embankment, Grosvenor Road and Millbank, with Cycle Superhighway 8 between Chelsea Bridge and Lambeth Bridge.

[ Sure this is only 14mph (compared to the ~20mph I'd do on the open road) but I'm also on a much heavier bike hauling plenty of clothes/lunch/stuff to and from work. It's still a good workout. The few times I've done it very early or very late I've come a lot closer to 20mph as I'm not slowed up by traffic.

The OPs journey might also be 2/3 rural-ish roads where you can make good progress, say closer to 25mph, and then the last 1/3 of the journey where maintaining that speed without interruption isn't so easy. ]