Understanding Interest Rates
Most interest starts off at a base number based on the current base rate set by the Bank of England, usually somewhere between 3% and 7% (September 2008 it is 5%). From there, the additional interest is adjusted based on your risk.
Now, the amounts may seem minimal. For example, a 20 point gain in your credit score only nets you a reduction of approximately a .125% drop in your interest rate. If you are borrowing very little (a couple of thousand pounds) the difference is only going to be 20 or 30 pounds over the course of a loan. Considering many loans can last as long as 10 years, that difference is minimal.
However, when you are borrowing a lot, in the range of 10's of thousands of pounds, every little bit your interest rate drops represents a dramatic difference in your total interest paid. That is why when you are riskier, the loan may raise your interest by what seems like an inconsequential amount (.5% or 1%) but the difference can be thousands of pounds over the course of the loan.
Each bank has its own calculation of a risk profile, and the base rate can change depending on the economy. But if the base rate is 4%-5%, then if you have no risk than you will get somewhere between 6% and 6.125% (it is rarely ever the base rate). If you are very little risk (say a credit score in the early 700's) then your interest rate should be between 6.375% and 6.75%. And if you represent a great deal of risk (650 and under) then your interest rate will go from 7% to as much as 9%, though usually no more than 8.5%.
Base rates are often higher depending on the loan, but the increments tend to be around the same amount.