I'm trying the '4 for £10' bedding/annuals from B&Q for the first time this year. The novelty is that the roots are wrapped in a permeable fabric (like a compost teabag!) that deteriorates underground to allow roots to grow through.
They've been really easy to plant, just pop them in the compost and your done, no mess!
You get 9 plants in a tray, so 36 for £10 (mix and match). I actually picked mine up for £1 a tray as they weren't shifting the deliveries quickly enough! Definitely worth seeking out a reduced section (ask staff for directions if you're brave 
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I have begonias, geraniums, petunias, trailing lobelia and snapdragons. I've also sowed nasturtiums to trail over the edges of my planters. You can sow these straight into compost outside, no need to start indoors.
Busy lizzies/impatiens and fuchsias are also nice. You can get packs of fuchsia plugs or Morrisons are doing bigger plants for £1 each, lots of varieties. Actually Morrisons is a good bet for bedding if you catch them on the right day. Ours has been a bit of a shambles this year, someone isn't looking after the plants/deliveries 
In mg experience slugs will leave geraniums and begonias alone, the others you may need to keep an eye on. I'm guilty of using the dreaded slug pellets to protect my plants in the early stages 