A transfer of a lease isn't the same as the sale of a business so TUPE will not apply ie it is the lease being sold, not the business. But of course there is nothing to stop the new business taking on Dave or Dave applying to them to take him on. I doubt a boycott would have any effect - people want coffee Dave or no Dave.

Personally I never use it - trains too crowded to manage a coffee and stand etc. And frankly better coffee available at the far end, should I want it.

Obviously it is in everyone's interest for London Overground to maximise all supplementary sources of income and this is such a one. I doubt they are doubling the rent but it is probably, within the terms of the lease, being increased.