First of all, Silverstone in April feels 5 degrees colder than it actually is and it will rain.
Silverstone is a big circuit and even a 50,000-strong crowd can look quite thin. At the beginning and end of the race people will congregate at the grandstands which improves things somewhat. Other spectators were very friendly although the lack of promotional stuff from the manufacturers was a bit disappointing. No big Porsche or Audi tents to look around during hours 3 and 4.
Congratulations on being one of the pitwalk beautiful people. It was manic last year and a lot of the LMP1 drivers were unreachable if autographs are your thing. Perhaps it'll be better this year but making sure you get in the queue early is important. Especially if you want to meet Mark Webber.
Personally I would recommend bringing an FM radio (the tannoys are rarely loud enough unless you're stood right under one). 6 hours is a long time to count positions and keep track of who's been in the pits and who's gone laps down. You look like an anorak but on the bright side you go away actually knowing who won the race. The broadcast is done by the excellent Radio Le Mans crew, who tend to bring some out special guests as it's their home round.
I'm told the security guards have been quite intrusive in the past but they were much better when I went last year.
Protip: Silverstone has doubledecker buses that circulate the infield all day. You can ride them nonstop like a hobo and I'm told that the top deck gives you a pretty good view of the race. But yeah, loiter behind the garages at the back of the paddock, walk down the Hangar Straight and see how many times you get lapped by the P1s, enjoy a motor race going on all around you. It's a good day out.