I just visited the F1 site and found that you can grab some decent discounts on weekend grandstand tickets with upto 20% off for early booking.
Valencia (European GP) has grandstand tickets for as low as £121.
Belgian (Spa) has got GA bronze tickets down to £147 (the same price as a single sunday race ticket.
German GP (Nurhburgring) has a lot of grandstand tickets for under £100.
Spain (Catalunya) has a 3-day GA ticket for £122.
Sadly no SIlverstone discounts but if you're wanting to go to the eurozone for an F1 experience these might be worth grabbing now. Nuhrburgring and Spa definately look better prospects but expect £20 a day parking if you drive!