I think it might be worth taking a close look at what is actually called the Currency and Banknotes Act 1928, specifically Section 12:

The penalty was changed to 25 pounds by s.31(5) & (6) of the Criminal Law Act 1977, and to 200 pounds by s.37 and s.46(2) of the Criminal Justice Act 1982.

So this is a very specific type of defacement, but something handwritten (e.g. slogans - or numbers representing how many notes are still in the group, as I remember used to bethe habit of sellers) is NOT covered by this.

And it could be argued that even machine engraving, or something "written" with a computer-driven plotter, is equally not covered by this, since it isn't"printing or stamping, or impressing by any like means", therefore not an offence (under THIS Act, at any rate).

I AM NOT A LAWYER, but do I feel confident in stating this, even though the Bank of England claim, at
https://www.bankofengland.co.uk/banknotes/using-images-of-banknotes

(My emphasis on the apparently erroneous words)
with a link to the section of the Act, at
http://www.legislation.gov.uk/ukpga/Geo5/18-19/13/section/12

This was reiterated by "a Bank of England spokeswoman" at
https://www.independent.co.uk/news/people/piers-morgan-five-
pound-note-did-he-break-the-law-good-morning-britain-
tears-up-rips-money-tv-video-a7244906.html

https://www.thestudentroom.co.uk/showthread.php?t=1791046
has a variety of comments, some less authoritative than others:

While checking this, I also found:
https://4.bp.blogspot.com/-bcjgUwZqUrs/TbBH_cqUsRI/
AAAAAAAAAtg/29H7tEf9xZ8/s1600/Lioncash.jpg ( from https://www.thestudentroom.co.uk/showthread.php?t=1791046)

A "Spocked" Canadian $5 at

https://www.bbc.co.uk/news/world-us-canada-31717210
https://news.bbcimg.co.uk/media/images/81372000/
jpg/_81372893_b_ivlqbu0aesbph.jpglarge.jpg
and the rather interesting saga at
https://fatprosemattrose.wordpress.com/tag/writing-on-money/
and too many more for this piece to hold.

Of course, Robert Burns' "Lines Written on a Banknote" was done in 1786, long before any such Act!

http://www.traditionalmusic.co.uk/robert-burns-lyrics/pdf/lines-written-on-a-banknote.pdf
https://www.bbc.co.uk/programmes/p03zspxb
http://www.robertburns.org/works/125.shtml

As it says at

https://www.exeter.ox.ac.uk/burnss-immortal-memory-celebrated-at-exeter-college/

There is also a reasonable amount of useful info (including that it was a Bank of Scotland 1 guinea note) at

https://www.lloydsbankinggroup.com/our-group/our-heritage/timeline/1695-1800/

By the way, I'd very much like to know it that banknote, or an image of it, survives!

To read the earlier E-Sylum articles, see:

IN OTHER NEWS: AUGUST 5, 2018 : Graham Short Releases Another Micro-Engraved(http://www.coinbooks.org/v21/esylum_v21n31a46.html)
ARTIST GRAHAM SHORT GOT BANK OF ENGLAND OK (http://www.coinbooks.org/v21/esylum_v21n32a30.html)