Keep a cat in London in a flat with "no pets" clause in head leasehold?

Im not familiar with how it is in London, I live in th US (Virginia). Most all places here for rent dont allow pets either. What I have always done is meet with the landlord and take a tour. If i really like the place, I talk to them earnestly, stating that I have just one cat, who is well trained and has no vices, and that I would be willing to put down extra for a pet deposit. It helps too if you have past references from landlords who can vouch that the cat caused no problems. This of course does not work anywhere there is a Property Management Company because they wont budge, but if it's an independent person, thay may allow the kitty. I would also recommend if they agree, make sure the agreement is added to the contract so they cant argue it later. Good luck!