Hi Carol,
It sounds as though you are planning to tow with an a-frame rather than a trailer?
In which case there is much to learn
Firstly, statement as issued by the Department of Transport
When an “A” frame is attached to a vehicle (e.g. a motor car) and towed by a motor vehicle (e.g. motorhome) we believe the “A” frame and car become a single unit and as such are classified in legislation as a trailer. As a consequence the car and A-frame are required to meet the technical requirements for trailers when used on the road in Great Britain.
So, yes you need the motorhome numberplate on the back, lights must be operated from the motorhome as must the brakes regardless of the 750kg weight rules (because the Smart is fitted with brakes they must work when its a trailer)
You should be aware that towing a car with another motor vehicle (motorhome)
in Spain is illegal - categorically and without doubt or the slightest grey areas!
Many, many motorhomes a-frame cars through Spain and very, very few get stopped but if you do, your only hope is that you can explain as nicely as possible that in the country of registration (UK) the towed car is a trailer when its on a special frame as in an a-frame and trailers are legal to tow in Spain.
Lots of 'knowlegeable' folks will tell you that a-frames are illegal in Spain so the decision to use one is yours.
We are be a-framing and understand the risks and are prepared to accept them - time will tell if we are being foolish
The worst case scenario is that we may get stopped, if so we may be required to uncouple and travel separately and we may get a fine -
or we may not.
Insurance? Make sure that your motorhome cover includes towing and that you Smart insurance covers being towed.
The company we got to supply our a-frame was TOWtal and there's lots of info on their
website.
Hope this helps