You'll get in the UK too. It's not a lack of understanding at all.
For example, I can see traffic off the ocean inbound to London that is currently doing 250kts at FL350. I know it's flying on a Mach number at the moment, but the Mode S is showing 250 knots so it's not hard for the pilots to keep it at that. If the traffic situation warrants it, and inbound to Heathrow it usually does, then I'll tell the pilot to keep the speed at 250kts from now. It's up to them to manage that through a combination of Mach number and deciding when to convert to IAS
In the case above, if you can't come back to that speed at that level then say something - we've always got a Plan B up our sleeve