Whereabouts in North London are you? If you're going to Jolleys, is that the Crews Hill one? Went in there once and saw the TINIEST hamster cage I'd ever seen >_< walked straight back out again haha.
Anyway, I go to Partners Pets in St Albans for most of my hay. Usually get the big bag (it's about 3ft tall, no idea of volume haha, just I have to drag it round the shop) of Dean's Farm hay at the moment. It's about £7 a bag and as I'm using it for bedding and food at the moment, it lasts a week.
If you are around Enfield way, Partners Pets is about half an hour drive from you!