The UK government has advised against all but essential travel to Sri Lanka after recent terrosrist attacks. Most attacks are deemed highly likely.

“Security has been stepped up across the island. A State of Emergency and night-time curfew remain in place. The Sri Lankan authorities have made a number of arrests in relation to the attacks, and security operations are ongoing, including controlled explosions of suspicious packages and vehicles, and temporary evacuations of buildings. However, the extent of any continued threat following the 21 April attacks remains unclear. On 24 April 2019, the Sri Lanka police issued a warning that mosques may be targeted by attacks, specifying ‘Aulia’ Mosques connected with the ‘burial sites of Muslim Saints’.

If you’re in Sri Lanka, you should:

remain vigilant, keep up to date with developments and follow the advice of local security authorities and hotel security staff;
keep a low profile, avoid crowded public places, large gatherings (including religious gatherings and places of worship) and any demonstrations. All of the attacks on 21 April were carried out either in places of worship or in places where foreign visitors were affected. Any further attacks may be targeted against similar locations;
avoid travelling during periods of curfew (other than to/from the airport – see below);
keep in touch with family and friends, ensuring they are aware of any plans you may have.”

More advice here here

If you have been directly affected by the attacks or need urgent consular assistance, call the British High Commission in Colombo on +94 11 5390639