If you are at NO point connecting to the Internet or installing untested software or untested USB drives you do not need to have up to date antivirus. That is assuming that the other devices on your own network (LAN) are all protected so that they can not become infected and then infect unprotected devices on the LAN.
It depends on what protection you have on your other network devices, BT Protect (AKA McAfee) isn't brilliant, in fact I've found the free version of Avast to be better. Imagine if you don't have an AV on your XP machine and you get a virus on one of your other network devices, your XP machine would be unprotected.
As has been said, Avast Free gives good protection and the Windows 10 on board virus protection Microsoft Defender is now getting good reviews.