Sorry - that information was completely wrong. She can drive to the prebooked test without road tax:
Schedule 2 - Exempt vehicles
22. - (1) A vehicle is an exempt vehicle [from VED] when it is being used solely for the purpose of;
(a) submitting it (by previous arrangement for a specified time on a specified date) for a compulsory test, a vehicle identity check, a vehicle weight test or a reduced pollution test, or
(b) bringing it away from any such test or check.
Vehicle Excise and Registration Act 1994