Sounds like more internet tosh, though I will gladly be proven wrong.
Section 1 Firearms Act as we are all so familiar with.
(1) Subject to any exemption under this Act, it is an offence for a person—
(b) to have in his possession, or to purchase or acquire, any ammunition to which this section applies without holding a firearm certificate in force at the time, or otherwise than as authorised by such a certificate, or in quantities in excess of those so authorise.
No mention of component parts and dies here or anywhere else in the act. No licensing requirement to purchase and all legal to hold.
There could be an offence under the Criminal Attempts Act 1981. The prosecution would have to prove the intent to construct the complete round. In this case the defence would certainly have their work cut out as possession of all the relevant bits when not entitled to hold the completed product is good evidence of intent. Is the act of collecting all the parts more than merely preparatory though?
As with everything legal it’s not straightforward and every case needs to be judged in its own merits.