There are two sets of rates — one for your own customers and another for 'trade' when you're subbing. And you'll probably use a different rate for backloads and part loads. And the rates you can charge are different in different parts of the country.
Most people charge a rate of so many pence per loaded mile (pplm), and the rate will depend mostly on the size of vehicle. You'll have to work out your rates as part of your business plan, but some of the topics here will give you an idea of where to start.