It seems to only be available from the US at the moment. Freshomes site says "The London Underground and Izola have collaborated to release the first ever officially licensed London Underground shower curtain. Printed on 100% PEVA, the curtain is virtually odorless and won't offend the senses with the odors often associated with plastic shower curtains."
Izola who produce the curtain have rightly cottoned on to the fact that loads of people from the UK will want to buy this. It's $29 but "Please note to all London residents that your order will be automatically converted into UK currency. The cost of the curtain including shipping is approximately £30 depending on your exact location. Please feel free to email email@example.com if you have any questions."