Police 'policy' is not law but the Police only need to suggest it to Parliament and it could be pushed through. At present the law states 'overall length' which can include screwed on items if the firearm is designed for them to be in place for normal operation. A screwed on suppressor would probably not pass that test unless the barrel was ported and normally covered by the suppressor. A welded-on suppressor is definitely part of the overall length.
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