A walker is known as a Zimmer frame in Britain.

Maybe it's jut me but I've always taken "Zimmer" to refer to the basic walking frames with either no wheels or two front wheels. I'd call the four wheeled thingies with a seat "Wheeled Walkers" or "Rollators".

I can't help with your query but wouldn't height adjustment be the main thing in getting upright? And maybe a physio (I think you call them physical therapists) could help you in getting the best posture?

Oh and we have one that sounds similar to Steve's. Its pocket under the seat is holding some of mum's gardening bits, secateurs, a trowel...

We could do with finding something better but again, I don't know what. It would have wheels that would ride the chippings and the uneven grass round the back better but could be no wider than the one we have.