The Royal Mail don't give you their best prices online.
Best get down the PO with your parcel, and get the post person to advise.
It should cost about a tenner, oi reckons, and you have a choice:
Airsure. Price includes tracking up to deivery, but is not signed for...
Signed for International, Normal small packet rate +£5.00 for signed for, but doesn't include tracking...
If you and the buyer want to take a minimal risk of no signature or tracking, the risks are minimal; it isn't Italy or Russia...
The new PO website is an utter joke! Ignore it!