You have to get a bit lucky on heading abroad with a 13 month ticket

If you got for an MOT within 1 month of it being due, you get it post-dated so could go away with upto 13 months. But if you get an MOT with more than a month remaining, you only get 12 months (always thought annoying you didn't get the 13 month ticket if having an early test, but c'est lá vie)
One option if 13 months would be useful and you are planning more than a year ahead is to get an MOT 11 months before you are due to depart the UK, then get a new MOT just before you go - that will give you the 13 months while away.