I find it truly hilarious that a driver from the U.S. would make any sort of statement about 20 mph being impossible to maintain. The standard speed limit in a school zone in the U.S. is 20 mph, and they are enforced quite zealously in many areas…quite rightly, I might suggest. To complain that a location or country has decided to enforce a 20 mph instead of a 30 mph limit is the height of entitlement. If you are having difficulty following the speed limit in the UK, you can certainly elect to use public transport instead. It is quite possible to travel around the UK without having a rental car, and the OP’s reluctance to accept that they broke the law and should face consequences suggests they perhaps should purchase a rail pass next time (which would open a whole new world of possible complaints, and likely would result in a much more sympathetic response to those complaints).