I think there was an AIC or similar about this a while ago, stating that the CTOT WAS TO BE met by the aircraft, no +10/-5. That is, as above, for ATC.
It is a fact of life that companys must strive normally to push 20 before CTOT at big/busy airports to allow for delays/queues.
If you ask politely, however, when you 'miss', I have always found LGW ATC will do their best, but there is no point in 'demanding' something that is not yours!