According to new Royal Mail terms and conditions documents dated January and Feburary 2013 it may now be the case that items valued at greater than £2500 are now insured up to £2500 (instead of not being insured at all). However, please do not take my word for it. Have a look at this, http://forum.tz-uk.com/showthread.ph...=1#post2645037, and follow the source documents to make up your own mind. I've not yet written to RM to get a definitive confirmation from them.
[This comment in square brackets added on 22/12/2013: I have crossed out the paragraph above because it was true all along that items valued at over £2500 sent by RMSD have no compensation whatsoever from RM for loss or damage. See http://forum.tz-uk.com/showthread.ph...=1#post2970558 and http://forum.tz-uk.com/showthread.ph...=1#post1987560 ].
Either way, it doesn't actually matter all that much: To fully and properly insure an item worth more than £2500 sent by Special Delivery you'll still need third party insurance of some sort. Check out the second message in the above thread, http://forum.tz-uk.com/showthread.ph...=1#post1987565, which lists the three most popular third party insurers.
That does seem to be the best option in terms of reliability and price. Check the above thread for some hints on how to sign up with them.