Hi there dhm - 'nuther fellow Britisher here - I see you posted this over a week ago, but since I see no replies and just in case you haven't found a source yet - personally I get my 'O' rings from the hardware section of a local department store ('Dawson's' in Clitheroe, East Lancasire). currently, but I've found them previously at some 'normal' hardware shops, although the ones that present more as 'ironmongers' tend to just look at me like I'm stupid and can't read, and yet usualy carry wooden broom handles LOL.
Also had luck in the past at B&Q type places, especially for cheap mixed blister packs of 'O' rings that should contain a few that will fit amongst the selection.
Car parts places also I've heard, but never bought them there myself.
For sizing I don't think I've once taken measurements to the shop - either I've got mixed packs anyway, or I take in the parts they need to fit on and tried a few until I find the size that works.
Usually really quite cheap - for that size I pay about 5p each, and most expensive size they carry are 15p each, and that from the dept. store place, and that's not a 'cheap' shop by any means.
Also, in a pinch and for not so critcal 'seals' like drip-tip to atty/carto/tank etc. would be, I've gotten by until I could get a proper '' ring by using a small, thin elastic band wrapped repeatedly round in the groove - works a like a charm.
HTH!
Be Well,
DV
