You can, but you cannot renew it. I tax two vehicles ONLINE on a monthly basis and when the MOT is due I am contacted and told that my tax will not be accepted without notification (from an MOT centre) that the MOT is valid. This was all introduced a couple of years ago as part of the anti terrorism legislation and is now catching up with everyone, not just the online users. Until the MOT centre has confirmation from DVLA that your motor is indeed noted as mot'd they will not hand over a certificate. The loopholes have been closed on the "vary" your tax and mot dates to avoid the MOT for as long as possible. Plus plod now has auto systems that check every number plate for valid tax, insurance and MOT, and the next step is the traffic wardens will get them as well. Believe me.