The penny black was issued in 1840.
They decided to change the color from black to red in 1841.
They were issued imperforate until 1854.
Maltese crosses were used to cancel penny blacks and penny reds until 1844. Then they issued numeral cancellations for each office.
In 1843, the London chief office issued themselves a set of Maltese crosses with numbers in the center - 1 to 12. You have the number 3.
Interestingly, the number 3 is the only one without a small cross at the top.
Not a lot of value in that stamp, as it has no margins around the design. Interesting variety, though.
For other examples, See:http://www.cgpostal.com/myvictoria/Slide71.JPGhttp://www.cgpostal.com/myvictoria/Slide72.JPGhttp://www.cgpostal.com/myvictoria/Slide73.JPG