Towing a car
Our view is that the safest method is to use a purpose designed trailer. There are devices known as A-frames which allow a car to be towed directly, the manufacturers of these devices will assure users that their use is legal. The Department for Transport have provided guidance on the subject of A frames and dollies: www.gov.uk/government/publications/a-frames-and-dollies/a-frames-and-dollies
The use of A frames may not be permitted by local legislation in some European countries but there is provision, within the EU, for vehicle combinations that are legal in their country of registration to be used when visiting another country where the combination is illegal for locally registered vehicles. However you may have to be prepared to argue the point if stopped by the local traffic police!
Towing a trailer
If you hold a standard car licence that was issued on or after 1/1/1997 it will have a category B entitlement, meaning that you cannot drive a motorhome with a MAM (gross weight) of more than 3500kg:
You can drive a motorhome and trailer combination of 3500kg or less (i.e the MAM of the motorhome plus the MAM of the trailer add up to no more than 3500kg) provided that the trailer MAM is less than the motorhome MIRO.
You can drive a motorhome and trailer combination of 4250kg or less (i.e the MAM of the motorhome plus the MAM of the trailer add up to no more than 4250kg) provided that the trailer MAM is no more than 750kg
In order to drive a motorhome (up to 3500kg MAM) and trailer combination outside the above limits you will need to pass a B+E test.
If you hold a standard car licence that was issued before 1/1/1997 it will have a category B, C1, B+E and C1+E entitlement, meaning you can drive a motorhome of up to 7500kg MAM:
You retain your existing entitlement to tow trailers over 750kg MAM, up to a total combined weight of 8250kg.