I found this one is a bit long-winded but seems to work:

^(?!BG|GB|NK|KN|TN|NT|ZZ)[a-ceghj-pr-tw-zA-CEGHJ-PR-TW-Z][a-ceghj-pr-tw-zA-CEGHJ-NPR-TW-Z]([ ]{1}|)[0-9]{2}([ ]{1}|)[0-9]{2}([ ]{1}|)[0-9]{2}([ ]{1}|)[a-dA-D]$

Allows for upper or lower case, and allows optional space between first two letters and numbers, allows numbers with optional spaces between groups of two, and an optional space before the last letter.