(AB is refusing to let me post this answer in one piece, so I'll try splitting it into 2 parts):
I think the answer to most of the questions you've asked is simply 'yes'.
Let's start, however, by assuming that you've not got an overdraft facility. If you've got, say, �50 left in your account and write a cheque for �200, the bank can do one of two things. Either they can simply 'bounce' the cheque (and charge you about �30 for doing so) or, they may honour the cheque and let your account go �150 'into the red'. (Depending upon the bank's procedures, this will show on your account either with a 'minus' sign in front of it or with the letters 'DR' alongside). This is known as an 'unauthorised overdraft'. There are three problems here. First of all, it was illegal to write a cheque with insufficient funds. Secondly, you didn't know when you wrote the cheque whether or not the bank would honour it and, lastly, the bank will almost certainly charge a very high rate of interest on the loan which it has effectively given you.
If, however, the bank has already agreed to an overdraft facility (as with the �500 limit you refer to) this means that you can write the cheque (or take money from a cash machine) knowing both that the arrangement will be honoured (i.e. no bounced cheques or problems at the cash machine) and that the rate of interest charged will be much more reasonable.
I hope that I've made my answer clear but just in case you're in any doubt: Yes, you can continue taking money out of the your account (either from cash machines or by using debit cards) until your balance reaches 'minus �500'.