I found this site: http://www.seedsofhealth.co.uk/resou...kombucha.shtml
near the middle it lists some commercial kombucha for sale in the UK. I don't know if you'd recognise the stores or brands though.
Someone else online said they have it at Sainsburys? (but when I typed Kombucha into the sainsbury website it didn't bring anything up)
I'm not brave enough to try making it myself. However, the husband has a friend who he is trying to partner with in opening a new pub in the next town over which would have Kombucha on tap ^-^ So I guess it can't be that hard. It seems to only take a week to fermement.