So, exposed to north easterlies and all that frequent soft Bristol mizzle and rain...with some morning sun, and in individual pots rather than window boxes or larger containers...I'd go for evergreens, smaller ones, a mix between variegated and green leaves. It'll be difficult to make an arrangement or combination of plants in pots that size, so you'll need to group the pots to create that effect. The only flowering things you could consider are really Pansies or the little Violas - there are also little cyclamen available, but they stop flowering around Christmas. Ornamental cabbages are quite striking, but again, past their best by Christmas, and because of the amount of rain you get, would need protection from slugs and snails. You'll find primrose/polyanthus/primula available as box bedding, but they don't start flowering till at least February.
Here's a few suggestions:
Pieris Little Heath
Skimmia japonica (they'll be in flower when you buy them in autumn and the flowers last practically till around February)
Euonymus japonica microphyllus aureus
Euonymus japonica 'luna'
Euonymus fortunei varieties - 'emerald'n gold' and others - these have a more spreading habit compared to E.japonica varieties
Topiaried or clipped Box
Dwarf conifers - a large garden centre should have a wide selection available, with variously coloured foliage from blue to greyish green to green to yellow, but check the height listed on the labels, usually means after ten years, to make sure its not a rapid growing very tall one.
You'll also find (in September) fairly cheap small versions of what will be large shrubs eventually - these are intended for use in pots and containers over winter, usually combined with ivies and pansies, so things like Choisya sundance, tiny versions of conifers, very small Skimmias, small hollies or Osmanthus heterophyllus 'Goshiki'. These will do for a year or so but will then need to be moved into larger pots, planted out in the ground or disposed of. Sometimes Ophiopogon nigrescens (looks like black grass) is sold, that's worth buying - it's evergreen and provides a good, low growing contrast to yellow or greyish plants. It will spread and can be planted in the garden later or divided and kept in pots.
Work out where you're going to stand the pots (best in groups, preferably with taller plants mixed with shorter ones, or groups of three of the same) and then work out whether you want two or three plants the same in a group, or a mix between what you buy.
If we happen to have a winter like the one in 2010, move the pots somewhere sheltered so they don't freeze solid.