Some of the more popular perennials are about over now, but there are still Heleniums, Rudbeckia, Leucanthemum and Echinacea on the go - a good selection of daisy type flowers. Later you can plant bulbs and next spring you can put in old favourites like lupins, foxgloves etc.
Meanwhile have a look at neighbours' gardens to see what is doing well. That is always helpful.

Wrong time of year to buy perennials really - any left still to buy will either be very large, and therefore costly, and out of flower, other than the ones Ojibway mentions above. In spring you can usually find cheap perennials in about March in the garden centre, when they sell them in small pots for about £1.99 each. I'm sure you don't want only daisy flowered types, so consider Physostegia, Tradescantia, Persicaria, Kniphofia, Alyssum, Aubretia, Primula/Primroses, Leucanthemum, Leucanthemopsis, Helianthemum, Campanula persicifolia, Campanula muralis, Polemonium. And remember that a cottage garden is virtually bare in winter...

Asters for late flowers are good but dont know if suitable for your area though?

Perennial osteospermums are fabulous daisy flowers and seem to do well at the seaside, but possibly east facing seaside in Scotland would be a bit too challenging for them unless you cover the crown in winter with one of those plastic domes that you can get in Homebase or Wilkinsons. My Mum used to have some perennial sunflowers that were pretty resilient. Marigolds should be OK - not perennial but self seed easily. Some of the tougher perennial geraniums would be OK - send a pm to Spritzhenry for suggestions of good varieties.