Be careful, if you want to go on a trip to Sri Lanka, you can't just buy a ticket and get on the first airplane. There are some things you need to do before you leave.
United Kingdom citizens who wish to travel to Sri Lanka must have passports valid for at least 6 months after the date of entry. A short stay visa can be obtained via the d’ETA (Electronic Travel Authorization) system, or it is still possible to obtain a tourist visa on arrival but this may delay you considerably. It's very easy to get a visa on the internet. Fill in the form carefully and pay the fee of $30 (approximately £19) on the site. A few days later you will receive confirmation by email that your ETA for Sri Lanka has been accepted. Once you have received it, you have 3 months to arrive in Sri Lanka and authorisation to stay 30 days in Sri Lanka.
There are no obligatory vaccinations for Sri Lanka. However, it is advisable to be vaccinated for Hepatitis A, typhoid, rabies and Japanese encephalitis.
Finally, if you anticipate renting a vehicle you will need an international driving licence. Which costs £5.50 from the Post Office
If you arrive in Sri Lanka without your ETA it's not the end of the world. You can still get a visa at the airport. It costs a little more (about £22). You need to show you have a return ticket. It is possible to extend your holiday to more than 30 days if you want to. In order to do so you have to go to the Immigration Office in Colombo, where you can obtain a visa for up to 90 days. You must do this at least 8 days before the end of your first visa.
Finally, if you want to go the northern province you must make a request in advance to the Sri Lankan Ministry of Defence.