Hmmm, it is as long as I can remember since I received any malicious email that was not detected by Google’s spam filters and sent to my spam folder, so it’s mainly that spam that would benefit from NCSC analysis in my case.
I did once try to forward a malicious email from my spam folder. It was a rather nasty (ie quite convincing) fake invoice/phishing attempt that had been ‘sent’ by a local firm of tree surgeons who’d recently done some work for me, and who’s account had been obviously compromised. I phoned them first to clear the way, then tried to send it back to them in case they wanted to see the evidence.
Gmail point blank refused to let me forward it since it contained a link to a known malicious URL, but can’t remember the exact nature of refusal.