This legislation applies to the use (and misuse) of distress signals:
The Merchant Shipping (Safety of Navigation) Regulations 2002, which came into force 1/7/2002, is the statutory instrument which brought into force in the UK the 1974 SOLAS Convention as amended - including Chapter V. MSN 1812 in 2007 amended some parts of the 2002 document, but not Regulation 35 which is the applicable one here. Most of the SOLAS convention only applies to large commercial ships, but parts of Chapter V apply to small, privately owned pleasure craft.
The regulations are summarised in a little booklet available from the MCA, or download from here http://www.mcga.gov.uk/c4mca/lrgtxt/solas.pdf
The actual text of Reg 35 is below ...
MISUSE OF DISTRESS SIGNALS
1. The use of an international distress signal, except for the purpose of indicating that a person or persons are in distress, and the use of any signal which may be confused with an international distress signal are prohibited