Where can I buy WHITE coffee filters in the UK? (and, to complicate things further, on a Sunday!)?

I'm a teacher, and I need to buy a bunch of white coffee filters for a science investigation. I didn't realise this would be problematic until I went shopping yesterday... I could only find BROWN (unbleached) filters. Ack! Any ideas where I could go on a Sunday to find some? I live in Buckhurst Hill, Es.