Unfortunately that's no longer the case as the act was amended in 2011. I'll leave you to make you own conclusions on how that happened!Thankfully the law is clear on this matter...
The Postal Services Act 2000, Section 104, Sub-section 2(d) provides that items going through the postal system cannot be withheld from the recipient until a charge is paid.
Now if Parcelforce could just operate within the law, there'd be no problem!
See section 35 of the link below:
Postal Services Act 2011
"A postal operator may detain the packet until the correct postage and any surcharge have been paid."
They are, however committing fraud by claiming on their invoices that the handling fee is levied by HMRC (false representation). I think the only thing that can be done is to report them to action fraud but it's unlikely that the police will do anything.