It is not necessary to be an existing bank customer to open a new business account, and you won't need to maintain a certain level of funds in your business account (although this may reduce potential charges).  Note that any bank will usually make credit checks on the owners of the business, so it helps if you have a good credit history.

It should take no more than two weeks to have the account set up and to receive a cheque book, paying in book and cheque guarantee card. However a credit card (useful to make purchases before your business has established a credit history) may not be offered immediately - it will take time to establish your company as a good credit risk.

Overdrafts, loans and debentures (a form of long term, fixed interest loan) will be offered on an individual basis.  Your bank manager will look at the amount required, what security you can offer, your cash flow statements and business plans, as well as your credit history as the proprietor/director of the business.

The British Bankers' Association (with Business Moneyfacts) provide an account comparison tool.  They also provide some useful tips on protecting your finances.