Vehicles used for specified purposes are exempt from certain sections of the Road Traffic Act.
"However, Section 87 of the Road Traffic Regulation Act 1984 exempts drivers of vehicles used for fire brigade, ambulance or police purposes from speed limits in an emergency. The Zebra, Pelican and Puffin Pedestrian Crossings Regulations 1997 give qualified exemptions from signals at puffin and pelican crossings to vehicles used for fire brigade, ambulance, national blood service and police purposes, and the Traffic Signs Regulations 2002 give similar qualified exemptions from other red light signals and keep right/left arrows to vehicles being used for fire brigade, ambulance, bomb or explosive disposal, national blood service or police purposes.
In no case is any distinction made as to whether a vehicle being used for ambulance purposes is private or not."
Lord Davies of Oldham in a written answer Lord Norton of Louth on 15th Jan 2004
S87 RTA 194
87. Exemption of fire brigade, ambulance and police vehicles from speed limits.[ (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 fire 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.
(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.
Notice the Lord was actually a little incorrect. The vehicle does NOT have to be involved in an emergency, merely that obeying RTA would be detrimental at the time.
Now the "technically" part.
Using your car as an ambulance should result in you not being prosecuted for speeding, but if you are not a response trained driver you could well be looking at a without due care or dangerous charge instead. The penalties for speeding are somewhat less in practice