With the opening of all non-essential retail on the 15th June, when will people be comfortable using public transport for non-essential journeys again? 

I'm assuming it's unlikely that there'll be a government announcement that bus/train enthusiasts can now ride to your heart's content - but if shops such as Primark are reopening, with public transport enforcing social distancing, then this means that people will likely be using public transport for non-essential travel (Though anecdotally many members of the public have been doing this throughout lockdown anyway).

Edit: I believe Stage 1 still advises against use of public transport wherever possible, with this not being mentioned in 2/3 so it is my assumption that the government are expecting people to use public transport again from June/July.