<Congrats on the G.B. find. I have literally looked at thousands of that stamp trying to find that error and yet to see one. Quite jealous!!>
Ditto. That's SG 362a, and if you don't want to look through thousands of copies on the off chance of finding one by pure luck there are several for sale at the moment on
ebay in the $150 to nearly $400 range.
A similar error occurred on a popular Danish stamp, the 1912 5 kroner Copenhagen General Post Office issue, (which was reissued 3 years later on paper with a different watermark). This had the name of the city (using the Danish spelling) as Kjobfnhavn, with an "f" instead of an "e", in other words, a missing serif as in the examples above. Even the normal stamp is highly desirable with a Scott price of $175 (in 2015), so the variety is icing on the cake. The opposite occurred (an extra serif) on some of the Danish parcel post, or postal ferry stamps of 1919-1941, where the overprint said "Posffaerge" instead of "Postfaerge".