By Amber Murray Blank Street UK has reported a huge increase in turnover this year after a rapid expansion into the British coffee shop market. The instantly recognisable mint-green chain, which operates around 30 stores in London and recently expanded to Manchester, announced a 517 per cent increase in group turnover to £11.4m. Blank Street’s net income for the year was £4.2m, “reflecting investments in store footprint, head office, and operations team expansion, as well as product development,” it said. The drinks company said the year was “pivotal” for growth and that its success has been d…