If you're not sure, don't risk it.
Ask the seller if you can leave it on his drive till monday or tuesday, wait till your cover note turns up, tax the car, then drive home safe and sound.
You could ask your insurer to fax or email you a copy of the cover note, then when you arrive on saturday to pick the car and documents up, you can drive straight to the nearest post office and tax it.
I have risked it before, but I don't think I'd do it again. You could end up with a hefty fine and points and it's not worth it for a 400 quid car.
Did you know that Britain has 20% of the entire worlds CCTV cameras? So you're even more likely to be picked up on camera in the uk than anywhere else.