Most companies are fine with people cancelling midway through pay monthly policies. Normally the only issue is that you'll not get your 1 years no claims bonus.
But as that doesn't seem like an issue... Then depending on the company you'll be ok to cancel but may have to pay a small charge. Either say, finish paying the month your cancel or a reduced figure generated by the company.
Best way to find out, daft as it sounds is just to ring them up and ask. (Remember cancellations will always try to make you stay as well! Prepare for a battle)