It's very unlikely that you'd be able to AVERAGE above 50mph unless you:
a) Go through the middle of the night
b) Drive at 150mph on the few occassions that the road is clear.
Can't help you there, have only trained-it to Wednesday before, but looks like plenty of street parking on google earth, provided you get there in good time.
Think I'm going to leave at 7.30.
Blimey, I'd be amazed if you managed to drive there in 3 1/2 hours. It usually takes me 5 - 7 (as a result of traffic jams on the M25/M1). Am going on the M11/A1 to avoid the M1 and expecting 5-6 (including an hour's stop).