I did a quick 'Google' on "medical bracelet necklace" and these are what I found on the first results page...
http://www.stickyj.com/meidbr.html (US) - Quite a variety, including some very expensive gold ones. You can order online and they will ship to the UK.
http://www.americanmedical-id.com/home/?usersession= (US) - Another varied selection, and they'll even sell you a jewllery cleaning kit! They also do 'sports bands' in case you don't want a bracelet. You can order online and it looks like they'll ship to the UK.
http://www.medids.com/ (US) - An assortment of medical items. You can order online but I couldn't establish whether they will ship outside the US.
http://www.medicalidalertbracelet.com/ (US) Small selection of items. You can order online but they won't ship outside the US.
http://www.beadin-beagle.com/medical.htm (US) Small selection, but some interesting designs. Can order online but will only ship to US postal addresses.
http://www.oneida-medical-jewelry.com/ (US) - Some interesting ideas, like medical tags to attach to your shoe-laces! Can't order online and they don't ship outside the US.
http://www.laurenshope.com/ (US) Nice selection including a few men's 'leather' bracelets. You can order online and they will ship internationally at extra cost.
Then of course there's always eBay
