So do we now have to resort to doing this to get what I think is the service they offered, i.e. choose my own seats - any seat, my choice?

I think it does. Presumably the reason why they wouldn't book you into a middle and aisle was because your reason for requesting those seats was your husbands limited mobility, if the reason they gave was related to health and safety? My mother also has very limited walking capability due to arthritis and osteoporosis and for her safety, and that of others on the plane, I always request assistance as well as medical seats for my parents. In the event of an emergency, I want the cabin crew to not only know exactly where my mother is so that appropriate assistance is provided but to also ensure that she is seated where she isn't going to be mown down in the rush by other passengers whose exit route she might be blocking however unintentionally.

Your husband has a right to this sort of assistance but more importantly all airlines T&Cs include them stipulating that they have the right to ask passengers who for safety reasons shouldn't be sitting where they are (eg passengers with mobility or strength problems sitting nest to emergency exits) to move. So if you had managed to book the seats you wanted they could well have still asked you to move once you boarded and potentially upset other passengers who'd have been forced out of their chosen seats to make way for you.

If you've ever seen any film footage of practice/training evacuations I think you'd realise how important all this was. Especially since research has shown that practice evacuations are reasonably orderly because people know that their lives aren't at risk. In an attempt to more closely simulate real conditions one research project offered a modest financial reward (£10 each from memory) to the first 50 people to get off the plane and the things that people were doing - including climbing over others who were struggling to get out of their seats - was shocking. If people are prepared to do that for a tenner imagine what they'd be prepared to do in the event of a real emergency evacuation when they thought that their lives were at risk.

SM