My friends' son is in a band. He has approached me to help him promote
a upcoming gig. He's suggesting stamping or writing his website
address on the £5 notes he gives out in change to people who buy
tickets.

Is it illegal to deface a British banknote by writing or printing
anything on them? What is the law?

Thanks

Not being a researcher I have just found what I could on defacing
banknotes  that might apply to your question.
Defacing basically means reducing the value of something (scribbling
in a library book), which writing or stamping on a banknote would not
do.  It would still be worth 5 pounds.
But, the definition in Fiji Island Law goes further, and Chapter II of
the law clearly indicates that is based on UK law, so I would think
that it would not be a good thing to stamp notes with advertising,
which obviously would show who was responsible.

Read below.
 
CHAPTER II - INTERPRETATION
General rule of construction
3. This Code shall be interpreted in accordance with the principles of
legal interpretation obtaining in England, and expressions used in it
shall be presumed, so far as is consistent with their context, and,
except as may be otherwise expressly provided, to be used with the
meaning attaching to them in English criminal law and shall be
construed in accordance therewith.

365. Any person who, without lawful authority or excuse, the proof
whereof lies on the accused, mutilates or in any way defaces a
currency note, whether by writing, printing, drawing or stamping
thereon, or by attaching or affixing thereto anything in the nature or
form of an advertisement, is guilty of a misdemeanour, and is liable
to a fine of forty dollars.

"... anything in the nature or form of an advertisement, ..."  !!