Does anyone know which is best for a driving day?

I did a bike track day at Mallory and it's a good track. Which is closest to where you live?

Mallory long circuit

Mallory is an easy circuit to learn and so you can spend most of the time getting to the limit of the bike/car, and if it is a bike, then you can try to get out of Gerrards FLAT in top!!!!!(assuming the mid corner chicanes not in use)

Prestwold Hall isn't really up to much IIRC, especially compared to Mallory.
When I did one there, they just did much of it on an area that seemed like an old airfield.

That's probably because Prestwold IS an old airfield! Mallory any day for me.

As stated above! Mallory proper "track", prestwold mostly on the airfield, and it's fookin freezing and open if anybody comes to watch you!