If you have a standard car license (B) this entitles you to drive any vehicle up to 3.5 tonnes. This includes most family campervans low profiles and some of the more compact A Class motorhomes on the market.
However larger motorhomes from 3.5 tonnes to 7.5 tonnes require the next step up, a (C1) license. You should bear in mind that once you drive a C1 category vehicle, rules and regulations vary. This includes speed limits and weight restrictions in some areas and on certain bridges.