Laws against false advertising on the other hand...
Not very extensively, but such is life.
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(1) A person commits an offence if he uses apparatus for the purpose of interfering with wireless telegraphy.
(2) This section applies—
(a) whether or not the apparatus in question is wireless telegraphy apparatus;
...
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Obviously, the "purpose" of the person using the anti-5G USB stick would need to be established. Given the law doesn't mention anything about having any effect on wireless telegraphy, presumably as long as mens rea is established the offence is made out.
There are also other potential offences around consumer misselling, false advertising, etc.
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[0] http://www.legislation.gov.uk/ukpga/2006/36/part/3/crosshead...