Most insurance companies determine that your car needs to have valid road tax and MoT for you to be properly covered, so driving your UK-registered car without road tax when overseas is inadvisable. It will also affect any breakdown cover you may have in place. Also, if the car is UK-registered then the DVLA continuous licensing rules come into force - you'd need to do a statutory off-road notification (also known as SORN) declaring that your car is not being used on the public road. Unless you are keeping the car permanently outside of the UK (in which case, you need to officially export it) then you will need to keep it taxed to stay within the law.