SS - I don't work at LGW but maybe they were of the belief that as the vortex doesn't start until the 757 nose lifts, they may have given you the option to go straight away believing you will be airborne before the 757 was and above all the vortex wake which is descending.
Also, it may have been offered (not insisted on!) if there was a strong crosswind which would have moved the wake. I understand why you don't accept it, but some probably do which is why you were asked.
Just a thought - and I very much doubt that they would delay you deliberately! If they are like me they just want to get rid of the traffic!!!
B-L