Here is a detailed document (James beat me to it!).
http://www.royalmail.com/sites/defau...July2013_0.pdf
Oddly, you are allowed to send with electronic goods even if not connected!
Lithium batteries are:
"Prohibited when sent on their own in the domestic and international post.
Allowed in UK and international post when contained in a device* but are
prohibited in Business Response, FreepostTM, PacketpostTM and PacketsortTM
Allowed in UK post when sent with a device* but are prohibited in Royal
Mail Tracked Returns®, Business Response, FreepostTM and all variants of
PacketpostTM and PacketsortTM"