I agree with Chicky that New Look polish is really good. Their gel top coat is almost as good as SV too.
I've been streamlining my polish collection and getting rid of the crappy ones. My favourite overall brand is China Glaze as the formula is really smooth and the colour range is great. My next favourite is Barry M, which are the best combination of value and formula. I love their new flat brushes too, so much easier to use.
I find OPI to be hit and miss; a lot of them are streaky and need 3 thin coats for total coverage. They have great reds and pinks though, and I love their mini bottles. I've been buying up Nails Inc from Poundland; I don't think it's worth anywhere near £15 a bottle, but for £1 - yes please. Their glitters are awesome, regular colours aren't so good.
I also have a few random brands that I buy for a particular colour. NYC Coconut is great for jelly sandwiches, Primark have some interesting one offs, Rimmel have a few nice quick drys, and Esscence and Claire's have some good glitters and top coats.
I've got about 150 bottles at the moment that I'm in the process of swatching onto sample sticks, to hopefully avoid duplicate colour purchases. I like to read UK nail blogs to find out what new colours or brands are coming out, and to see swatches before I buy.