If it was for IBS get a doctors note to confirm your wife suffers from IBSand below is that bit from the road traffic act but I dont see where it says that this can be by Joe public
87 Exemption of fire brigade, ambulance and police vehicles from speed limits.E+W[F1(1)]No statutory provision imposing a speed limit on motor vehicles shall apply to any vehicle on an occasion when it is being used for [F2fire and rescue authority], ambulance or police purposes, if the observance of that provision would be likely to hinder the use of the vehicle for the purpose for which it is being used on that occasion.
[F3(2)Subsection (1) above applies in relation to a vehicle being used—
(a)for Serious Organised Crime Agency purposes, or(b)for training persons to drive vehicles for use for Serious Organised Crime Agency purposes,as it applies in relation to a vehicle being used for police purposes.(3)But (except where it is being used for training the person by whom it is being driven) subsection (1) above does not apply in relation to a vehicle by virtue of subsection (2) above unless it is being driven by a person who has been trained in driving vehicles at high speeds.]