Scott.as for the op... it is your own land (or you parents) which being classed as private means you can fire your paintball gun, air rifle (up to .22 unless you have a valid firearms certificate) and airsoft marker...
scott
Unless you do know what you are talking about (which you don't), you shouldn't really comment on something as specific as Firearms Law.
First, calibre ("up to .22") has no influence at all, it is simply the muzzle velocity that it develops.
Secondly, a Firearm certificate may allow you to posses an air rifle that produces over 12ft/lb of muzzle energy but in order to have been allowed it, you will have had to specify where you intend to use it and have that land cleared by an FLO (Firearms Liaison Officer).
It is unlikely that the average back garden would be allowed, both on safety grounds and on "what you need". Meaning the average garden would be both too small for an FAC air rifle and (because of that) would not require the power of an FAC air rifle.
There are other inaccuracies in your post as well.