In general the component parts of ammunition do not count as ammunition until they are assembled to make a useable round.
The exception to this rule is a bullet designed to expand on impact.
Under the legislation these are theoretically prohibited and would need a Section 5 authority from the Home Office.
However the Deer Acts require the use of soft nosed ammunition for shooting deer and it is good practice to use it for vermin control as well.
For this reason there is an exemption under the Firearms Act which allows the purchase of expanding ammunition on a Section 1 firearm certificate provided you have a condition to that effect.
The Home Office guidance is silent on the matter of how many expanding missiles (bullets), as opposed to complete ammunition, you may buy.
Some police forces do specify the number of bullets that can be purchased but unless they have done so it is probably a matter of your dealer playing it safe.