It massively depends.
I have two possible commutes to work - one is about 8 miles and one is nearer 9.
The first one is two and a half laps around Regent's Park - where I will average about 15mph. But there are only four sets of traffic lights on a three mile lap and very few pedestrians.
The other is from Wembley to Camden - this is always a good couple of mph slower. Because I'm filtering through traffic, there are tonnes of traffic lights and there are loads of idiot pedestrians. It's a little bit more hilly, but not much.
On the Brompton Worlds
@cougie uk mentions (we did it the same year) I averaged around 18 or so (I can't now find the Strava log). SO it's a bit of a guess. I know I did the first lap at 22.5, then got progressively slower.
But all of those routes are fairly flat. I did the virtual BWC during lockdown which is a 19 mile route of your choosing. I live in the Chilterns so 19 miles anywhere around here take a fair amount of climbing. So that ride was about 13.9 mph.
I tend to find that hills slow me right down as I'm simply too heavy. I'll normally be in the top third of all riders (even on my Brompton) on the flats, but in the bottom 10% on hills.