Comprehensive list is here:
www.royalmail.com/personal/help-and-support/Tell-me-about-Restricted-Goods
If you send something in their Restricted Goods list, and Royal Mail notices it, they will confiscate, and sell at auction.
Case in point: £300 suspension unit for bicycle. "Disposed" of by Royal Mail, was confirmed to have been sent for auction. Full thread here:
singletrackworld.com/forum/topic/royal-mail-have-binned-my-rear-shock