as said above canon or Nikon would be the preference of most here. They are generally good cameras whether you go for entry level or high end and it is really just down to preference of which you prefer to use. Look for a good condition used body and lens somewhere like ebay. I cant comment on Canon models having always used Nikon myself but you should be able to pick up a Nikon d90, d5100, d5200 etc for between 200-500quid realistically. All of these will allow you to buy additional lenses, will allow long exposures for underground photography and are all pretty good at high ISOs for handheld stuff.
Really you should be explaining what photography you would like to mianly use it for and also the price range you can afford then people may be able to better advise!!