The cheapest way to borrow is usually through a secured loan. In the UK this is usually through a legal cgarge made on your property (house). As the lender has a very good chance of getting their money back if you should default, it means that they are able to offer the lowest rates of interest - currently as low as 4% APR. This type of loan is normally called a mortgage. In certain cases it is possible to have more than one loan secured on a property ie a second charge. This depends on the amount of equity in the building (ie the amount over and above the mortgage you already have), your earnings and other factors.
How much can I borrow?
Different mortgage lenders have different lending criteria. Usually it will be in the region of 3 times a single income or 2½ times a joint income. Mortgage lenders will usually also look at your monthly outgoings to see if you can afford to make the payments and any other loans or outstanding debts that you may have.