You’ll need a VE103 vehicle on hire certificate to show you’re allowed to use a hired or leased vehicle if you’re driving it abroad. Read more on GOV.UK article “Taking a vehicle out of the UK: For Less than 12 months”.
You can get a VE103 for a fee from the:
AA
British Vehicle Rental and Leasing Association (BVRLA)
RAC Motoring Services
Road Haulage Association (RHA)