At PRYZM we use DespatchBay - this is then linked through Parcelforce or Yodel if you choose. However I always send it through parcelforce, as you can then take it to your local post office and hand it over the counter, meaning that you don't have to wait in for a collection(and there is a surcharge if they collect less than 4 packages).
Costs around £7+VAT for the courier service for 24hr delivery, but you cannot make one-off payments - so the cheapest amount you can put in your account is £50+VAT.
This service has been superb, within about 5 years of using this service I'm yet to experience a failed delivery, or hear of any real major damage via transportation (touch wood).
Just to add, they also provide the customer receiving the package sms updates and easy rearrange options.