There was another very similar question some time ago - maybe someone will remember how to find it.

Would the pensioner be agreeable to you putting weedkiller on her garden or strimming it every so often - it might be easier to offer to maintain it than to keep fighting weeds which will keep on seeding into your plot!
Its difficult to think of any ground cover that won't get weed seeds growing up through it and I'm wondering if some low shrubs might be easier as you could glyphosate round them with care every so often. You could underplant them with spring bulbs that would disappear before most of the weeds grew so you could add weedkiller in summer without damaging them, or hoe round them or hand weed, which would be better but more work! If you want suggestions we would need to know whether your soil is heavy or light and whether it is sunny or shaded.

I once used Glysophate weedkiller on grass at the edge of a concrete path to save work trimming it. It killed the grass 3 feet wide !

As Steragram says, need more info - in sun or shade, how big are the areas you want to plant, and how big is your garden? And what part of the UK are you in?
That said, I'd go for large and small shrubs - because they're woody, you will still be able to use weedkillers like Weedol 2 and glyphosate on any soft green weeds which appear because they don't kill woody plants, so as long as you avoid the leaves on the shrubs, they will be unaffected. Quite what your choices are is down to the answers you provide to the questions above.