I disagree with Giorgio - as would the UK civil owners of the Victor, Vulcan, Jet Provosts, Hunters, Venoms, Meteors, L39s, Vixen etc. BAe would love to sell you a Hawk and the RAF don't really have a say. Admittedly it wouldn't be armed, but you can't have everything. The biggest problem would be getting the thing on your licence so you could fly it. Also, if you could afford the aircraft, the insurance wouldn't be an issue. However, as the Hawk is apparently a rather nice aircraft to fly, a bit of trading with "Campaign" would be possible. However, the Hawk is a bit quick for a first type, so I'd recommend a PC9 (ex. Aussie AF as they have rather nice glass screens) or a "steam" ex-RAF Tucano.
You're making some confusion here: all the types you mention have been purchased after retirement of the type, not from new ! As such they are ex military aircrafts for the entity who sold them (it doesn't matter if the type might still be operational somewhere else). A lot of them didn't even come from the UK, so had nothing to do with the RAF. In the UK it's possible to have these certified and fly them and the same occurs in other countries.
But a Hawk from BAe is a different story: the Hawk is a product commissioned to BAe by the MOD, the sale of any new Hawk from BAe to anyone, civilian or military, is under government control. Unless the government allows BAe to sell to civilians, a direct sale of a new Hawk will not happen, full stop !