I don't think the decision by News Corporation to put the US edition of the Wall Street Journal on sale in London is going to disturb the Financial Times unduly. It's more likely to hurt the Wall Street Journal Europe.
In truth, London's business community is being super-served already, with City AM and the paid-for titles. A fourth entrant is unlikely to expand the readership. Anyway, as one never tires of saying, the WSJ and the FT are very different animals (and not just in the colour of the newsprint).
The WSJ gives a US perspective. The FT gives a British and, crucially, an international perspective. In the short term, even if the WSJ's journalistic culture is changed, the FT's regular London audience is unlikely to drop it in favour of the WSJ.