Hit your local poundland...
In the last year or so my local one always has some really decent brands (revlon, sally hansen, opie, etc) that cost upwards of £8 in normal shops, for just a pound each.
There's a range of colours - seems to change all the time - and at that price they are cheap enough to experiment with whilst still having decent varnish.
I am currently sporting a dark purple/aubergine-y colour which is very nice for winter. I've also got a couple of nice neutral corals that work really well, a bright red, a bright pink, a nude, a silver, a metallic soft green, a pale blue and a peacock blue. All for the price of a single decent nail varnish in Boots.
I've been pleasantly surprised in summer by the soft green and the peacock blue colours - they work really well with tanned feet (not quite so good when I'm pale in winter though!). And I like the jolly red/pink/coral depending on what I'm wearing in summer too.
Depending on what colour I've got on, in the summer I update them quickly by just doing an overcoat where possible before jumping in the shower - the solvent in nail varnish dissolves out in water so the nail varnish dries under water, so you don't need to hang around before putting stuff on your feet which can be handy.