Website to decode the area letters in a VRM
XXnnaaa - look up XX in the current list and
ynnnaXX/aXXnnny - look up XX in the historic list.
Even though the regional issuing offices closed in 2013, DVLA still issues regional letters to dealers.
Interestingly Tx was also used for Scottish plates circa 2007, so you might see some starting with T that are legit. Ones starting with J are all private plates, reserved for DVLA selling. Ones starting with X are sometimes seen too and these are similar to the older style Diplomat plates you used to see/sometimes still see in London. I have a nerdy interest in number plates, don't judge
Similarly, in the previous format, ?OO and ?PU are Chelmsford regs., usually a good a indicator of Dagenham 'factory cars'.
The number of ?PU's in our reg. list is quite noticeable, including evidence of possible sequential 'block' registrations.