The majority of my colleagues are aware of the difficulties speed restrictions can impose on flight crew (with regard to descent profile, turbulence, tail winds etc.) and try to take this into account. There is obviously, however, a vast difference in performance between aircraft types flying in/out of the UK, and we simply don't have enough detailed knowledge about every type of aircraft and what the pilot can/can't do with it. Therefore, the logic most people seem to use is to ask for what would be easiest and if the pilot is unable then he/she will say so (and most pilots seem to). Then we can sort out something different. This happens regularly when asking for level and speed restrictions at the same time etc.
Obviously there are different objectives the flight crew are trying to achieve in each aircraft (whether it be to save as much fuel as possible, or get to destination as quickly as possible), but when the airspace is busy everybody has to fit into it safely, and this will mean you could be assigned a speed of anything between 250 and 320kts typically to ensure aircraft remain safely separated. If you are unable to reasonably comply with the restriction imposed, then please say so. That is aftera ll what we'd expect you to do.