Why would the english not participate? It's U23 anyways. Or am I missing your point?
Its complicated.
At the Olympics, the United Kingdom competes a single team - including athletes from all four of the home nations - England, Northern Ireland, Scotland and Wales.
In soccer, each of the home nations compete individually. There is no 'United Kingdom' Football Association under which a UK soccer team could compete at the Olympics. No doubt one could be created for the Olympics, but the fear amongst the home nations is that if that were to happen, it wouldn't be too long before there was pressure for them to compete as a single team in all competitions and no-one wants that. So they simply don't compete in Soccer at the Olympics.