You need to check the wording of your insurance policy carefully. Mine includes this sentence; "Your vehicle must have a current MoT certificate if applicable". (That seems to contradict the first sentence of Paul's post).
Assuming that your father's insurance certificate has similar wording, the question arises of what is meant by the words "if applicable". It seems that your father's insurers are trying to say that it means 'if the car is three years old or more'. However, a better interpretation would seem to be 'if the law requires it'. The law does not require a vehicle to have a current MoT certificate if it's being driven to a pre-booked MoT test, so there would seem to be no reason for your father's insurance to be invalidated.
As stated, start by reading the exact wording of the policy. Then, if it's the same as mine, write to the insurers stating that a current MoT certificate was not 'applicable' because it was not a legal requirement for the journey being undertaken. If that doesn't work, register your complaint with the Financial Ombudsman:
http://www.financial-ombudsman.org.uk/consumer /complaints.htm
Chris