Cherryface - if things look shrivelled up by mid summer, its one of two things - either you're buying the wrong sort of plants or you're not watering them enough. I have a number of tubs which moved from my last house, although, now, with a big garden, not got so much time for the tubs! One of my favourite and a very easy to manage one that should like the sun is a flatish pot which I have planted up with very small alpines - a tiny pink, thrift, small thyme, tiny scabious, very small geranium etc. Although this is small, it has quite a lot of plants in that flower over a long period so there is interest for quite a long period of time.
Another favourite is a hedgehog holly bush (slow growning, with lots and lots of spines on each leaf) - at the moment this has lovely yellow crocuses coming up. I also have a large tub with a 5' variagated holly in (although I think that I might plant this out this year). I like box bushes and you can clip these into interesting shapes - look good all year. I have a tub or two with herbs in (sage, thyme, rosemary - garden too wet for these to thrive), and I have a tub with a bit of trellis on the back with a clematis, a jasmine and some ornamental grass in the front to add some height. All of this lot need more watering than the alpines one though. Generally speaking though, I find it easier to keep perenials alive in tubs and consider some underplanting with annuals (geraniums, bizzylizzies if slightly more shady) to add a little more colour over the summer.
Overrun - I have garlic already in, although you can still get some (and onion sets) for spring planting). Based on the last couple of years, most of my veg will initially be planted in pots inside (runner beans, tomatoes, marrows, courgettes, pumpkins) and I will only sow lettuces, carrots, beetroot etc in the ground when its warmed up a little. Not planted any pots up yet though as quite far north and don't want to plant things out until beginning of May ish.