I just checked the London by-laws online. Unfortunate, but true - 2 ft and max two pets (with a few exceptions) You can download the PDF file online at
Check under Public Health - Animal Control By-law - PH3 (15 pages)
Some of the animal/pet restrictions in that city are the craziest I've ever read. If it weren't such a serious matter, I'd say it was an April Fools joke for some or the laws they have.