Even better, we have a copy of Brunswick 02370 with an LT sticker (tax in force 13 Nov. 1947 – 8 Apr. 1948) attached to older stock with tax code NP. So NP must have been used not 1953-1955, but before November 1947.
Could NP be Decca/Brunswick's version of the ST tax code (10 Apr. 1946 – 12 Nov. 1947)?
The UK Brunswicks between 1953 and 1955 in our database seem to use N exclusively (and "Made in England" has moved from the bottom to the top of the label).
One of the few exception so far seem to be images 410214 and 410215, which use the older label design with "Made in England" at the bottom and NP as the tax code (another example: Brunswick 02803 NP repress). Maybe this is a very early 1953 repress?