UK Passport
A visa is not required for UK Passport holders.

Driving Licence

International Driving Licence
(especially if you have the old type (no picture) licence.
Available from the AA @ £7.50.

Insurance Green Card
Now not normally required - check with your insurance company

Log Book

MOT Certificate
And if it's near to the expiry date, renew it before you leave, in case you get delayed.

European Vehicle Recovery
Check if your policy covers Hungary and Romania

Travel Insurance
Recommended, but the choice is yours...

Not your vehicle?
Letter of authorisation to drive vehicle, if the vehicle you are using doesn't belong to you.

MAPS
A set of road maps would also be useful (Try the Map Shop, Upton Upon Severn), unless you use Sat Nav (or should that read especially if you use Sat Nav!).