You can put a vehicle through the ITV at anytime, but the next one is still due in either the two or one year, from the date of that test, depending on the age of the vehicle.
I am reliably informed that one is NOT PERMITTED to drive a vehicle on a road if the ITV has expired.
I spoke to my insurance company and asked if I could have the vehicle taken to the ITV free by grua, under the policy. They said I would have to pay for the service.
Thus, as I was going away for 3 months, and the ITV would have expired whilst I was away, I had it done a couple of months early.
Shell, Foxbat answered your question in an earlier post about re-testing when a vehicle fails.