Guess we can ignore this, then, those silly Customs and Revenue folks:
HM Revenue & Customs
Sailing your pleasurecraft to and from the United Kingdom
3.1 Do I need to fly the yellow ‘Q’ flag?
If you are arriving directly from another EU Member State there is no need to fly the ‘Q’ flag.
If you are arriving from outside the EU (and this includes the Channel Islands) you must fly the ‘Q’ flag, where it can readily be seen, as soon as you enter UK waters (the 12 mile limit). Do not take down the flag until you have finished reporting to the customs authorities, as described in sub-section 3.2 Failure to comply will make you liable to a penalty.