You are probably lucky in having no frost and being 3 miles from the sea I would think salty air is not a factor so you will have a large choice of hardy plants to choose from.

Of course if my assumptions are false then the choice will be much more limited.

What size are the pots and in what position (e.g. south-facing wall)

Why perennials? Why not some shrubs, hebes and pittosporum come to mind immediately, followed by arbutus, choisya, phormium, santolina, tamarix . . . . Phil

perennials can be shrubs or herbaceous . i would go for a mix of shrubs and you cant go far wrong with a few fuchsias. what about bulbs, there are a range of bulbs/tubers for every month of the year pretty much. dahlias, daffs,lilies, gladioli etc.
many sages/salvias are good too.
bt it will depend on size of pots as already mentioned.