But an "old rare" coin is often worth over £100 ...But, yes it does look like that something like an Old Head penny would be covered.
I think the RM's latest definitions of money and valuables is fair. A package containing current legal tender is more susceptible to theft and is is only covered by special delivery which makes sense. The T&C for tracked delivery states "If you breach this obligation [of sending valuables which include current legal tender] then you do so at your own risk and, to the fullest extent permitted by law, we exclude all liability to you such in circumstances (and, to avoid doubt, we will not pay any compensation to you in such circumstances). Hence, I think it is hardly reasonable to expect any compensation when you are posting a modern coin using tracked delivery.