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!!
As above, I picked up a used Nikon d5300 from ebay, with ridiculously low shutter count for about £450 with a similarly used 18-105 lens (which isn't good) I add bits to my setup as and when I can afford it, I now have a battery grip, loads of batteries, SDcards, a tripod, Nikon 35mm lens, Sigma 10-20 lens (which is good) and loads of other bits and bobs - probably taking about 8-£900 spend over maybe 4-5 years.
Previously I used a hand-me-down Canon 450d with kit lens, but as others have said it's not about necessarily about the camera - mine's good enough without being ridiculously costly, and has served me well.
That said, you can buy a 450d for about £100 nowadays