Apparently Sri Lanka, being a former UK possession, is a rare example of the UK socket/plug being in use. There is another type also in use and that is rather like the now discontinued round pin plug from the UK. You may find it difficult to get a suitable adapter for the latter in the UK with airport shops being shockingly expensive if indeed they have any. Seeing as they seem to use the current standard UK type I would imagine your best option would be to buy one in Sri Lanka if you find you need it.