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Dont do it, You wont have canonical issues from just have London repeated in a H1. But you will hava problem if you stat down the reaod of cloking

I believe the recommendation is to have the title only be strictly the business name (with no keywords in it.)  As long as all the offices are real locations and you've verified them either by phone or mail, I don't think that's ever been an issue.  You may want to do other things to differentiate the locations from one another, such as unique photos, unique names of staff members and so forth.  And if each location has its own page on your website that's even better. -Dan