Well, I don't come to Gatwick very often but here's my thought.
If, more often then not, pilots are told that they can keep high speed when they ask for it... the pilots can start thinking: well, they don't grant us high speed unless we ask for it. If that's the case, they will continue to ask for it. (logical isn't it?)
Two solutions: you and your colleagues reply "no" all the time when asked. After a while pilots will know that it's useless to ask for high speed and stop asking or (second solution) when high speed is granted more often then not: change the rule so that A/C can keep high speed unless told otherwise.
I bet that no pilot will bother you asking if they can please reduce to 250 kts when they pass FL100.
Regards,
Sabenaboy