By Jessica Frank-Keyes Sadiq Khan is set to warn Brexit has hit London’s jobs market, claiming almost 300,000 fewer positions are available in the capital thanks to the UK’s exit from the European Union. Khan will make the claims at a glitzy dinner at the Mansion House this evening. Half of these losses are in construction and financial services, down 81,000 and 92,000 in London respectively, new analysis released alongside his speech suggests. The mayor of London is also expected to highlight the claim that the average Londoner was nearly £3,400 worse off last year due to Brexit, and the aver…