I have very recently sent and received 2 rifles via Parcel Force 24hrs Service and insured upto £500-1000, with no issues at all.
Being that they now clearly state in the Policy that ammunition in question can be sent I don't think there is an issue.
However if the said item or any item is lost in the post and one goes through a Claim Procedure then Parcel Force will ask for Proof of Purchase**. If this can be provided parcel force are duty bound under their loss&stolen Policy to provide Full Compensation of the lost item to the value to what it's insured to.
Very rarely things go missing with Parcel Force on their 24/48 hrs service.
"If your not prepared to be wrong, you'll never come up with anything original".