"Given that the UK had been part of a single market which allowed for enormous freedom of movement, its access to all that talent in Europe is now in jeopardy."

I don't think UK Govt will jeopardise access to talent with unprogressive immigration reform. UK tech startups need not worry on this front. And just look at USA as an example. Despite VISA issues for foreigners, Silicon Valley still thrives and still has reasonable access to a global pool of talent.