Unless you have access to a commercial carrier contract (via your work, for example), you're stuck with Parcel Force. I'd use a 24hr service - it spends less time in their system. And make sure you wrap well and then box, using plenty of packing tape. Ensure you write "Do NOT use knives" on the outside and last, but not least, take out their (optional, I think) insurance to the value of a new replacement.
Best regards
Paul
Take a look at the <A target="_blank" HREF=http://www.royalmail.com>Royal Mail website</A>. There's a postal calculator which lets you specify weight and compensation value, then shows you the various options available, together with precise costs.