I can only speak to the foreign degree aspect as I am in the same situation (and in your department at KCL, funny enough). Still, from what I understand, if you are currently finishing the MA, you will have to convert the results of your degree with a Department of Education accredited conversion service —I would recommend World Education Services— and then convert it a second time upon graduating if you get accepted. Otherwise, you would just have to convert it once if you have graduated. As for selection, I believe it is not an issue, though I cannot say that authoritatively, and for the clearance, it's not an issue, just a bit more difficult to fill out.