Webbley
I now use this service for almost ALL of my UK deliveries, it saves a trip to the post office and most of are received the following day!
Parry
I used Home Delivery Network to send some tyres I sold on eBay. I got the buyer's address slightly wrong but after contacting Home Delivery Network's customer services, they were happy to change it free of charge. Unfortunately the tyres still ended up going to the wrong place but after making another phone call, it was eventually rectified.
Generally I found Parcel2go's price pretty competitive and their website was fairly easy to use. I would recommend them to my friends.