Another news source reporting UL to MAN (and LGW):
News
But, in order to meet the growing passenger demand, the national carrier plans to add seven more flights to England alone by the end of the year on top of 14 that it now flies to Heathrow in London every week. The seven new flights are to Manchester and Gatwick, SriLankan sources said
Also stating the reason is mainly tourism orientated, and not so much local traffic.