They helped me out of a sticky situation in 2010 when I attended the British GP (they kindly agreed to sell me a camping ticket even though I didn’t buy my event ticket from them).
Unfortunately their delivery costs are quite high and there doesn’t seem to be an option to arrange collection but if you’re happy to pay extra and prefer the peace-of-mind of having something in your hand before you make the journey, I wouldn’t look anywhere else.
They also have a reward scheme. Each £ you spend accrues 1 point, 100 points = £1. I guess similar logic would apply to the € or the $. It’s not a lot let’s be honest, but it’s a nice gesture.
I have always bought my tickets from here- fair prices, early delivery (got this year’s tickets a month ago already) and very helpful if your plans change and you need delivery to a different address. http://www.spagrandprix.com