Good evening,
If you go to
https://www.cyclingtimetrials.org.uk/find-a-course you can find a time trial course near to you, hopefully.
As these courses are just routes on public roads you can ride them yourself whenever you like.
Once you have found a course, you will find that it has a code something like K46/10. You can then search for that course code and local club's time trials results to see how you compare. As nowadays many casual triathletes also do these events so you won't just see times set by dedicated TTers on expensive dedicated TT bikes.
I agree with the 20/25mph comments above, but 20mph is faster than it sounds so don't be surprised if you get onto the road and find it initially unachievable.
Bye
Ian