I would definitely stick with Royal Mail and avoid the likes of UPS, DHL, FedEx, TNT etc.
I used both FedEx and DHL back in November to send an A4 envelope each time. The envelope was going to quite a remote part of Korea. Delivery with FedEx took 9 days and DHL took 3 days. Each envelope cost �50 to send!
The cheapest way to send a parcel is via surface mail. As far as I'm aware, if you go into your Post Office, by default they will assume that you require airmail. Ask specifically for surface mail and you should notice a dramatic difference in price. I would advise you to ask about delivery times though. Don't be too surprised if they tell you it could take a month or more!