Compare to the registration sheets. The west border of the southwest corner is very distinct. Letters should look very similar in placing and shape. Also, the weak top line in the northwest corner (star) is a good help in identifying.
https://www.gbps.org.uk/information...phabet-I.phpIt should be from one of the plates found under the sets that refer to those that have "Alphabet" in the name (see lefthand side), or reserve plates.