Given (official*) UK speed limits are always in multiples of 5, using the 8/5 rule would be quicker and work just as well.
* There are occasional places where private property has speed limits with weird factors, but they're so rare as not to worry about. (the one example I remember is when I used to cycle to work and took shortcuts through a sewage plant that had an '11 mph' speed limit, why? who knows.
