Probably a bit late to answer, you'll have already done your purchasing!

A plant that deserves to be grown more widely in pots is Anagallis - long season of deep blue flowers about 1.5cm dia. Use in the same way as lobelia, but it's a lot less easy to reduce it to a pile of hay by forgetting to water it. Easy from seed.

Diascia are also good for a long season of many flowers.

It's a good principle to buy plants in the smallest pots possible - they'll usually grow away very well. Fuchsias and pelargoniums/geraniums are easy from cuttings.

Small ornamental grasses/sedges are a good option for when there's not a lot else around - again, easy from seed. Look for mixed sedges (eg from Chilterns)

Don't forget late summer stuff. I tend to use Calendula (pot marigold), eschscholtzia (californian poppy) and nasturtiums - various shades of cream/yellow/orange/red which will take you through to the frosts. All easy from seed. Also Ipomea (Morning glory) as a late season climber - flowers only last a day but there are lots of them. I grow mixed seed, so one morning the flowers may be mainly blue, the next purple, the next pink and white. Not an artistically tasteful planting, but it's fun.

If you like the tomato idea, think also of chilli peppers (may not get very hot in Cheshire outside), strawberries (there's one variety which has deep pink flowers), alpine strawberries, or even a mixed planting of purple-leaved french beans and lime-green lettuces.

Seeds vary in how long they can be kept for, but most can be kept at least one year, so you can build up quite a variety without huge outlay each year. Keep them in a plastic box in your fridge.

Don't forget scented things as well - night scented stock and nicotiana are easy and powerfully scented. Very good in pots because you can move them around to where you spent most of your time.