The Firearms Act 1968 (as amended) prohibits the possession of expanding ammunition.
However, Section 5A(4) allows a condition to be added to a firearm certificate for possession of expanding ammunition for:
Schedule 2 of The Deer Act 1991 prohibits the use of any ammunition, other than soft-nosed or hollow-point, for the killing of deer.
It will therefore be seen that if you use a weapon for deer stalking, you will require an authority on your firearm certificate to acquire and possess expanding ammunition.
Similarly, expanding ammunition should be used for the killing of large vermin, such as fox, in order to ensure a quick kill and prevent unnecessary suffering.
Section 57(1) Firearms Act 1968 defines a "firearm". See What is a firearm?
It should be noted that this definition includes "any accessory to any such weapon, designed or adapted to diminish the noise or flash caused by firing the weapon".
This "accessory" is known as a moderator or, more commonly, a silencer. So it will be seen that to possess a moderator you will need to have the authority on your firearm certificate, the same as you would for any firearm.
As with any other requirement for a firearm you will need to satisfy good reason.
The most common reason for requiring a moderator is, in the destruction of vermin, to prevent additional quarry from being frightened off when the first round is fired.
Every application for a moderator, as with any firearm, will be treated on its own merit, but if you feel you need one, then it should be included on your application form, in addition to any weapons requested.
You cannot legally possess a moderator unless you have the authority on your firearm certificate.
Some rifles have built-in moderators and if you feel you need one of these then your application should read along the lines of:
" . . . .22 rifle with sound moderator . . . "
It is an arguable point that moderators are not effective with full bore rifles which project a missile above the speed of sound. Although diminishing the sound of the discharge, the missile then breaks the sound barrier with a sonic crack thus defeating the object of the moderator.
Any ammunition fired supersonically will create a similar problem.