into the UK from the south, you are more than likely on an ATC speed restriction anyways to stream you as there are so many inbounds - esp to the TMA. If you cannot fly that speed iinstruction, you MUST tell ATC.
If ATC do not put you on a speed restriction (which can be a fly greater or less than restriction), then technically you are free to do what speed you want, as long as you meet the 'level by' restrictions.
It would be nice to be told if economical speeds are being used, although it is a RADAR environment, so a controller should notice!!