I can't imagine its fun to open your way through that pile.
My mum has her and dads presents and her siblings gifts to us at hers and that's a large enough pile (15 ish gifts per person) and it feels like a lot
But I can see how you could have so many if you buy stuff for the year.
Clothes
Shoes/wellies
Nightwear
Underwear
Stationary
DVDs
CDs
Books
Toiletries
Toys
And I don't believe it's £500 per 100 presents. Ok, you can get decent stuff stationary, toiletries, sweet wise from poundshop and also things like packs of 5 cars etc.
But I suppose you can get socks for £1, underwear for a few quid, books and DVDs etc for £2-3 if you shop right. But I'm not sure I'd find Christmas enjoyable if I spent all year looking for bargains and preparing for it.