well the closer a gym is or the happier you feel their the more likely you are to continue, so why not work out how you plan to get to these places, car/tube etc and do a cpl of test runs to a few gyms, go have a look, see how long it takes and which one you like the feel of etc. only way to know...
If you are looking strictly for MMA in North London, one of the best places I've seen is the MMA clinic in Angel, literally 3 minute walk from Angel tube stop.
Also Bandogs MMA runs out of Rooneys boxing gym and thats pretty central so might be worth checking out.
Hope that helps