The law say's you can legally drive it to an MOT that has already been booked in and that they can confirm,AFAIK you cannot even drive it for work to be done etc.If it needs work doing then it must stay at the garage until it is completed and a new MOT certficate is issued.I am not 100% about this as is OH's area rather than mine,but am pretty sure it's something along those lines.
They now have the ability to check if a car has an MOT without having to pull you over and ask etc,as all MOT's records are now held on a central database that garages have to enter all info in at time of car coming in for MOT,so if you drive it now or if it fails and you drive it away they will know!!
Also makes your insurance invalid,so TBH probably not worth the risk of driving it for the sake of waiting until friday.Pain in the butt I know,but better to play it safe I would think.
Fingers crossed you manage to get it in sooner!!