I've done Silverstone a few times, but not since the Mansell era. You have like road access to the track these days!
My advise is, take pots of cash as everything is hugely expensive. There are plenty of famers fields that open up for camping and as you'll be around from the Thursday or Friday you should have no trouble finding a site just driving around the era. The farmers dont ask for much, they just move their cattle out and say "there you go, it's a field". Facilities are not exactly laid on and you've cow pats to dodge, but it's cheap! If your staying for 3 days though you might want to check out a B&B for one night to clean up or something.
On the way out of the circuit after the race it is massively busy, I gather they have improved the roads since I last went - but one way around this is after all the days racing is done head back to the carpark - unpack the barbeque from the boot of the car, and have a picnic with all the other racers loitering around and doing the same. As otherwise getting out is just a very long sit in traffic.
Also if you can, try and arrive early enough on GP day to get to a car park that is nice and near the silverstone exit ! One year my brother and I left the car at the campsite and walked the last few miles, it was a nice day and the track was easy to find we just followed the traffic jam.
Now in terms of where to watch from, back in my day you could get an infield pass for £13 and I gather you could go into the paddock back then, but I dont think they allow that any more. I did the infield thing once trying desperately to get a good vantage point as Mansell mania saw to it that it was hellah packed, the paddock walk about was pointless so near to the start of the race - so if they still do any infield pass type things dont do it unless you get there early enough to take advantage of it.
I like to watch from around the Luffield complex, you get to see a lot of the cars and that part of the circuit is usually home to some incidents and some overtaking.
I gather watching from the Vale is also a prime spot although i've not done that personally. Other parts of the circuit are not so spectator friendly, although you may have more personal space around you!
Always leave somebody in your group holding your perch! When the race starts suddenly the stands are teathing with people.
Dont be shy to pack some ear plugs, people might think your a twat, but when the field come by the first time they'll be looking at you going "I wish i'd thought of that".