The bundled Moby list already knows many British forms (judgement,
defence, programme) but still flagged others: honour and its inflections,
colourful, colourless, rigour, glamour, prioritise, privatise,
synthesise, paediatric and a few more.
Adds those base forms to assets/spelling/en-extra.txt (inflections come
from the existing rules) and a regression test. A separate en-GB
dictionary chosen from w:lang is still open on the issue.
Co-authored-by: Claude Sonnet 5.5 <noreply@anthropic.com>