Recommended purchases

[Brian]

Reynard,
I'm going through the process now.
The form is not a big deal, the pain is getting someone to countersign the form and endorse the photos you need.
You don't need to prove the proposed use for the gun, you only indicate the sort of thing you have in mind, eg. clays, rough shooting, driven game etc. and have an idea of where you can legally do this, so if you've already joined a club that would be best.
You can expect to pay around £100 for the cabinet (a good supplier is guncabinetsdirect who have free delivery and fixing kit on some models). The gun you choose is a big subject but go for a 12 bore; traditionalists prefer side-by-side doubles but the clay shooting brigade prefer over-unders. Pumps and semi-autos are restricted to two shells in the magazine under the SC rules but both are OK for clay shooting. All of these can be obtained for under £200 second hand but the cheapest of all is the single and can cost under £100, which with an ejector is still a good rough shooting gun but is unsuitable for clays due to the rapid second shot requirement in some disciplines. Chokes are a another mine-field but on a double you can't go wrong with three quarters and a quarter, semi-autos and pumps often have multi-chokes so you can change them for the sort of shooting you are doing and single barrel guns are mainly full choke as standard. Full choke gives you the longest range as it constricts the spread of the shot whilst true cylinder has no constriction and the shot spreads quite widely, then you have quarter, half and 3 quarters in between.
Cartridge choice is a subject in it's own right and very fascinating too.
Hope that helps a bit and just go for it.
Brian