The Women's Suffrage movement kicked into high gear in Great Britain with the formation of the Women’s Social and Political Union (WSPU) in 1903. The WSPU specialized in highly visible and often violent campaigns such as large parades, shouting down speakers, stone-throwing, window-smashing, hunger-strikes and arson. Most historians agree the overall effect was to set back the cause of women's suffrage.
Defacing currency was one of the less-violent, nevertheless illegal, activities. British large pennies were usually stamped, letter-by-letter, with VOTES FOR WOMEN, and this was when a penny actually had some value.
Genuine suffragette pennies are somewhat rare. If a piece is genuine, it usually brings a good price on the auction block:
worn: $20 US dollars approximate catalog value
average circulated: $30
well preserved: $80
The trouble, of course, is that a particular coin might be recently made, rather than stamped back at the time of the WSPU. A professional coin authenticator, and most professional coin dealers and experienced collectors, can probably tell the difference between a genuine piece and a modern-day fake. Modern day fakes are worth zero.