No issues with mot, as long as it's secure, if it's in the car i.e hatchback you need a seald gel type battery really, you dont want the gasses leaking into the car.
I haven't done it myself but my Rover has the batt in the boot and it looks a simple enough of a job to do. Just remember to use 0awg cable. And as said above make sure it is secure and you shouldn't have a problem...
i was thinking of doing this in the trans am just one of them jobs i though about and never dont it its easy to do you can get a kit for it just with big wires to run rear to front
nothing for the mot the battery is not part of the test
but there is no upside for doing it unless you have a light weight race car