If a player has a choice between London and Dallas or London and Chicago, he will probably not pick London. That means London will always get players barely hanging on in the league. If they do draft great players, those players will flee back to the US once their contracts are up for negotiation. Only those desperate for a job will stay.
London will be permanently disadvantaged as a team, making it non-competitive. Once the novelty wears off, fans will realize that their team can never win and London, having much higher costs, will become a money loser.
They might as well name the team the London White Elephant, because that is what it will become: an expensive team with little prospect of winning or turning a profit.