No VISA is required to enter Romania for all UK passport holders. Please ensure that your passport does not expire within 6 months of your travel date. If so, its worthwhile getting it renewed well in advance of your travel date.
Don't forget that any children will also need valid passports.
For non-UK residents, please check the VISA requirements well in advance - there is often a long waiting list to obtain VISAs so dont leave things to the last minute!