Hi George,
Although you can buy AA membership at the roadside, I'm pretty sure they don't include relay. You would have to get membership prior to your journey (I think membership with relay is around £100 now).
Even then I think they would refuse you relay on an untaxed, unMOT'd car.
It would be quite obvious to them what you are doing.
Personally I wouldn't risk it.
Have you considered van or trailer hire?
Oh, and you'll need a stack of yellow pages to cover all the areas between Poole and Glasgow

Anyway, best of luck, hope you get the car, it looks like a bargain at the moment.
Paul
.