Rhodesia
"Illegal stamps"

As a reaction to the unilateral declaration of independence of Rhodesia on November 11, 1965 British postal authorities regarded Rhodesian stamps issued later as "invalid" although this was not justified by UPU rules.

Formular aerogramme sent from SALISBURY (24 JAN 1966) to London, UK for philatelic purposes. The postage paid was 
6 d.
 
The calculation of postage due in the “Rhodesia case” did not follow uniform rules. According to information compiled on stampboards.com the rules were:

• 8 Dec 1965 - Apr 1966 : Double deficiency surcharges based on correct postage required less the value of any legal stamps present
•  
• 17 Feb - 8 Oct 1970 : Calculate the correct postage less the value of any legal stamps used; double this value; convert the result to sterling at 1c = 1.4 d

The stamp on this airletter was regarded invalid and the aerogramme was taxed with double the deficiency (12 d = 1 s). A boxed green handstamp was applied: “INVALID STAMPS / USED / POSTAGE DUE ...”, “1 s” endorsed and a 1 s UK postage due stamp affixed.

Another aerogramme from Rhodesia from a later period and some background of the "illegal" stamps can be found here.