Hello Peter
Well done for doing the exercises. My Dad (89y/o didn't when he was your age and regretted it - 'til he could no longer remember regretting it
).
The a huge choice out there in your price range. Forgive me for making a huge assumption, but I'll take a punt you're not looking for a hardass full suspension mountain bike (MTB) for hooning down mountains at breakneck speed?
Have a rummage on here as there's a lot of discussion on exactly what you're asking. General advice is buy local if you can (for aftersales support). And try several different bikes first - essential in my book. You might prefer a step through frame rather than a cross bar.
If it's leisurely cycling on mostly tarmac, you're probably better off with a hub drive, than a crank drive.
In your price range is the Wisper 905 Torque, which was no.1 on my list when I was looking last year, but Woosh advertise on here and they'll have a selection that'll suit you. The Oxygen S-Cross is held in quite high regard, or for a top spec R&M, there's one currently advertised on here which you could afford:
http://www.pedelecs.co.uk/forum/threads/riese-muller-blue-label-roadster-touring-for-sale.28790/
Anyway, welcome aboard, and happy e-bike hunting!
Edit: Presumably not far from you and as good a place as any to start:
http://electricbikesscotland.co.uk/our-range/
Gaz