[ninalanyon]

> It is a defence to say you forgot it,

Do you have a source for that assertion?

[ChrisKnott]

It's only an offence if you "knowingly fail" to provide it - https://www.legislation.gov.uk/ukpga/2000/23/section/53

s3 makes it clear that if you plausibly claim you have forgotten it, then the prosecution must prove this is not the case (i.e. you still know it) beyond reasonable doubt.