callum91
I'm an ATCO, not a pilot. I would never give take-off clearance until I'm sure I'll get the required wake turblence spacing, for example if I need three minutes I'll probably start giving the take-off clearance at 2min30s as by the time you've read it back and rolled, the three minutes will have elapsed. This is the way I was taught at college and it seems to work. I know some pilots will use a stopwatch to time the gap from the previous departure and not start their roll until the required time has elapsed. This is fine from my point of view.
It's useful to know that ATCOs consider the wake turbulence to be significant from the time the nosewheel lifts, or for a heli from the time the weight of the aircraft if transferred from the gear to the rotors. We also try to appreciate that a light heli will produce more turbulence than a light aeroplane and might try to give you a bigger gap if we can.
If you haven't done so already, you can send one of these
http://www.caa.co.uk/docs/33/SRG1423.pdf to the CAA. I think it's mostly for research, but it helps towards a better understanding and therefore safer operation for everybody.