Hi, Katie here!
I second Kittie in saying you get what you pay for when buying corsets although if you're at at the 'budget' (and you'll probably know why I've put that in inverted commas) end of the market you should come out with a good basic corset for around £100. I've worked in the fetish industry for over a decade now and over the years have stocked Axfords, Vollers and Snobz, all of whom offer good corsets starting at around £100. Go for a well known name and you should be okay. Go for What Katie Did and you can send anything back for a refund so there's very little risk!
If you have the cash to splash then Eternal Spirits are great for a fancier corset - as is Miss Katie.
Camden is a good hunting ground - as well as Fairy Goth Mother there is The Arc (200C Chalk Farm Road just before the Stables Market bit) who we stock, and there's also Pink Fluffy under the arches who have a good reputation.
We'll be at the Skin Two Expo at the beginning of October and if you're after a corset it will be well worth going. Dark Garden from San Francisco will be showing - they make beautiful corsets, as will Velda Lauder. Puimond (LA) has exhibited the last couple of years but I'm not sure if he's there this year.
www.skintwo.com - Skin Two
Katie