The HXP I have, has a flat base solid lead core bullet and is loaded with Olin Ball type powder, its boxer primed not berdan primed.
Near as I can tell its identical to Winchester "Whitebox" generic .303 ammunition.
Is there a type of MkVIIz that does not in fact use the MkVII bullet?
Only Britishmilitary .303 loaded with other than cordite that I've heard of was MkVIIz and MkVIIIz.
Also I've found that HXP ammo was produced under contract for the British government in the 80's but this appears to have been 7.62 NATO rather than .303.