Hi Richard,
What governs the towing capacity of the towing vehicle is its plate capacity that will be shown on the VIN plate. You can tow up to 750kg without having to use a braked version so with an approximate kerb weight with fluids (say 30kg ) = 650 kg you will be well within the limit.
Having said that ensure you have a trailer board that works, if your dolly actually lifts the front or rear you will need to use a flashing beacon as well to be street legal as it comes under lift and tow regulations.
Technically you cannot use dollies on the motorway ( although you do see many of them ) as they were designed as ' breakdown units ' and should only be used to take a casaulty to the nearest garage for repair. Hence not to go passed a slip road.
I was involved in this market sector in the UK. It has always been a grey area as some of the terminolgy used is open to interpretation. Let common sense rule drive safely within the speed limit and be aware of the longer stopping distance.
The Laird of Gozo
