Royal Mail's radical method

Surprises for a female colleague this morning at the mail : the slow arrival of a postcard sent by another colleague during her holidays in England. The first surprise is the timing : 2 months from March, 3rd (date of message) to May 2nd. Here is the second one :

Front picture of the card (scan of a photocopy).

A quiet large sticker announces that the sender was a little cheap in the franking which couldn't reach the airmal rate. Conclusion : the card followed an "alternative service".

On the back, a "First Class" stamp that allows the ending of a less-than-100-grams prioritary and standardized letter inside the United Kingdom. It valued 32 pence between 21 August 2006 and 2 April 2007 (34 pence since).

The tariffs are easily found on the Royal Mail's website and show some severity toward letters to Europe (I think so) :
* there is no tariff to send a post card to Europe ;
* so, you use the tariff for a letter less than 20 grams by airmail : 48 pence,
* and the surface mail (by ship) is not available for Europe : 46 pence.

It miss 12 pence that the Royal Mail decided not to get back through the postal administration in the country of destination. A two months delay and a big sticker seemed to be an adquate punishment in the eye of the British post.

Note :
Thanks to Céline who let me photocopied the card before the unstick the message.

Question :
Do one of you know what can be the "alternative service" use by the Royal Mail ?