I know a bit about this after doing couring for the last 6 months.
Although some companies will be a little cheaper than others,the best way to get the best price is to be flexable and not in a hurrey.
For example,let the courier collect it at a time and date that suits him/her.
The best way to get it cheap is if the courier has a few days at least to get a collection of items all going to a similar area.
The other end of the spectrum is one item that needs to go the same day.
Same day couring is normally at least a pound a mile (while goods are on board)
So 100 miles=£100 aprox.
But if he/she can have multiple drops in a roughly similar area the cost will be so much less than that.......