Despite occasional repetition, and publication in UK stamp magazines, it's amazing how many people* still don't know the rules as I wrote up in 2002:
Norvic Philatelics Royal Mail Horizon Postage Labels webpage wrote:
Collectors of British postage stamps have known for years how difficult it is to get very-fine-used (VFU) copies of any stamp. Ordinary letter rate stamps have a machine postmark, higher value stamps on packets have ugly black rubber postmarks, either round(ish) for letters and packets or rectangular for parcels. Postal items bearing these labels will not be cancelled at the sorting office. But - if the postage is partly paid by stamps, then those stamps must be cancelled at the branch when the label is affixed for the balance of the postage cost. And that cancellation must be done with the branch's steel counter date stamp - the very thing that collectors of VFU want!
* Sadly with so much reorganisation in Post Offices, the closure of crown offices and location of even main offices in branches of WHSmith, Lloyds Chemists etc, this applied also to many PO counter personnel. Sometimes reminding them works, sometimes it can lead to heated discussions. Sadly sometimes even inland Special Delivery and overseas Signed For items leave the counter uncancelled
Philatelics GB stamps info: http://www.norphil.co.uk
- also /catalog for our ecommerce site, blog.norphil.co.uk, shop.norphil.co.uk and Ian_norvic on twitter