As in 10 mph?
Yes.
Remember some of the riders are as young as 10, some are arm-powered
[1], and some are on Mildly Inappropriate Cycles™ or gas-pipe creations not designed for speed. Sometimes more than one of the above. Yes, things get serious at the sharp end of the fast race, but most people are participating as a social event or experimenting to see if their latest bit of fettling has improved performance. It's mostly about Fun.
This year I'm riding a touring trike with a seat that doesn't fit properly, just to be different, which has absolutely ruined my performance, and is entirely in the spirit of the BHPC.
Mind you have to remind yourself that the length of these events means you would be able to work much harder than you do on a long ride. So speeds should be higher than the averages riders put out when on the road.
This.
Combined with the fact that it's a smooth tarmac track devoid of potholes, gravel, oncoming traffic, muppets in wankpanzers and so on. You can corner as close to V
max as you feel able to without flying off, which does wonders for your average speed.
It's also pretty flat (the lower loop has a Dutch Hill-Climb, the oval is flat) compared to real life, and since you're going round in circles, you tend not to get an omnidirectional headwind.
[1] Though a couple who came to Darley Moor were Paralympic standard, and therefore A Bit Quick.