The Martin Baker MB5 does seem to be the most common answer when questions like this turn up. The three main problems, aside from the crash of the MB3, though were that they seem to have been a fairly small and undercapitalised firm, the weren't part of the Air Ministry's 'magic circle' - during the slump in the 1920s and 30s they limited their orders to a limited number of manufacturers to try and keep them afloat, and that James Martin was apparently a major perfectionist. Now the first two could possibly be got around if they were able to raise more investment money, either commercially or from a Lady Houston, of Schneider Trophy fame, type figure. Baker apparently taught a large number of rich and famous people to fly at his air school at the Heston aerodrome before leaving to start the company so that could be one possibility. Martin's perfectionist tendencies however are a somewhat more intractable problem. Best I can think of is that a larger aircraft company, either as a full takeover or a large minority shareholder, or the government becomes involved so that there's someone to keep reminding him that 'perfect is the enemy of good'.
You've also got the problem to identifying a need for it. The Air Ministry was already funding the Spitfire and Hurricane for their fighter requirement, with the belief that 'the bomber will always get through' still being generally accepted a long-range escort could well be seen as superfluous.