How narrow is it, this border, from front to back, and what's in front of it? A path, or grass? Because the lowest maintenance, easiest care planting is actually shrubs, maybe with a few bulbs and some ground cover plants, but if the border is very narrow, then your choice of shrubs is restricted by that. Lavender, whilst lovely, is relatively temporary, because it gets very leggy and usually needs replacing in five years...

Do you want tall or short plants ? There is so much to
consider, good that you are planning.
Is it windy where you live ?
The ground sounds very rough, plants like their own little
space with a bucket of J.I.No.3 in that size hole to put their roots into. It contains nutrients and would retain moisture.
A few bags to use as required would be an asset.

Hi thanks, the border is narrow against a concrete path so maybe something that softens the edge but doesn't intrude too much. Had Hidcote lavender before, but old and leggy!
But still love english lavender [and 5 years better than annuals for me thats a plus, but any suggestions gratefully received. Budget to another thing to consider I guess.

Any chance you could measure it - narrow might mean a foot, 2 feet or (hopefully) 3 feet...

I think Dianthus is a good reliable, weather proof, perennial, in different shades of pink and red. Suitable for a narrow border by a path.

The roots of the young plants, usually about 4" deep. would
settle happily in a planting hole 7" deep, so only half a bucket of John Innes No.3 would be needed for each plant.

They flower profusely, would just need dead heading to tidy up. Cost about £1.85 per plant.

Hardy geraniums are hard to beat for labour saving and there are so many different varieties to choose from.
A few hellebores would largely look after themselves apart from removing the old leaves in winter and daffodils and snowdrops will cheer the spring. Butterfly gladioli should last a few years if its not desperately cold in winter. Variegated marjoram makes a good edging and is practically indestructable and bees and hoverflies enjoy its pinky/lilac flowers. A clump of Rudbeckia is trouble free and lights up the autumn in a sunny spot. Wild gladiolus communis byzantius are lovely trouble free dainty flowers and a few low growing evergreens (everybody will know I'm going to say Euonymus fortunii...)to brighten up the winter.

Oh help, Just realised you said TWO....