I thought I would drop this link in for future reference as it’s a well thought out and well presented guide to plating the GB QV 2d Blue.
It’s published over on Stampboards and written by emason (username).
This is just a short simple guide to the early twopenny blues issued between 1840 and 1858 and how to tell them apart. They were printed from only six plates (1 to 6) and determining which of these a particular stamp was printed from varies from the straightforward to the tricky. Having scans of the plates is useful, but it is possible (most times) to plate them without resorting to comparing the check letter positions.
Image Credit: emason