If you’re a British citizen, you will need to apply for an Electronic Travel Authorisation (ETA) prior to travelling. This can be done via the Department of Immigration's official website. Whilst you can also apply for this upon arrival, your journey through the airport will be much faster and smoother if this has been authorised beforehand. You will normally be granted access to the country on a tourism visa for 30 days. For a stay beyond this, or to work, you will need to apply for a visa. In addition, you must make sure your passport is valid for at least six months after your date of arrival. All passengers must complete an embarkation form too, which can be done online before you travel.
You can only get a direct flight to Colombo from Heathrow Airport (LHR). This route is operated by SriLankan Airlines, with an average journey time of 10h 25min. There are more options for indirect journeys, which might be more convenient for you. For example, from Manchester Airport (MAN), you could fly with Qatar Airways in around 13h 55min, which will most likely require a transfer at Doha Airport (DOH). From Edinburgh Airport (EDI), you could fly with British Airways in about 14 hours, with a stop also likely at DOH. Alternatively, Gatwick Airport (LGW) offers flights with airlines such as Emirates in about 13h 20min, with a stopover normally at Dubai Airport (DXB).
If you want to hire a car in Colombo, you will be able to access many car hire agencies at Colombo Airport (CMB). Many of the car hire companies, such as SIXT, offer a meet and greet service, whereby you provide your flight details and arrival time, and a staff member will meet you upon your arrival and take you to your vehicle. Remember to bring a current driving licence, passport and a credit card that’s in the primary driver’s name.