How about the police decide how to deploy a shared resource based on police primacy at a public order based major incident? It isn't really a loss of resources if you join the specials and aren't available for some major emergencies. You can still do street patrol, reassure people etc. and deploy to many major incidents. A train crash, unexploded bomb, Hazchem incident or bomb scare would most likely not need you for the Fire service, as they either don't require that big a fire service response or would mainly need firefighters with specialist training the Fire and Rescue Authority won'y pay for for retained firefighters. It would of course be different if a full time officer was a retained firefighter, as that would be denying the force a trained body for use at major incidents, since there is a budget for so many full time officers and no more.
Depends on the brigade (service) as to how they utilised their retained crews. Some will train their retained crews in specialist areas. However, I think to say the flexibility is there to choose what you would be doing that day in terms of wearing a police or fire uniform is somewhat misleading. If a retained firefighter commits to a number of hours and availability at a given time, he or she needs to meet that commitment or they wont get paid their retainer.
In terms of attending incidents like train crashes or hazchem incidents, the Fire Service will generally have a major role in these, and retained firefighters have been a key part of that response, as well as ensuring fire cover in the surrounding areas should it be required due to the number of crews committed elsewhere.