In answer to both queries, the answer is that there are NO prerequisites, other than as has until recently, the British MOD has always tried to buy British. Not only to safeguard British jobs, but also to retain control of the intellectual property and to stop the proliferation of advanced combat machines, equipment and software from getting into the hands of their potential competition and to minimise any increase in the threat risk.
The UK has always had a very vibrant Arms industry, where innovation has always led the way. That was until of course the collapse of the Soviet Union and Warsaw Pact, after which time as far as I’m aware, the UK ceased to invest heavily into future proofing the British Armed Forces. Bit by bit funding has been reduced and projects cancelled, forcing Governments of all persuasion in to panic buying off-the-shelf kit when they suddenly realise there’s a shortfall.
So, Imagination and Innovation…. It can be old kit that’s been constantly upgraded to meet requirements (longer range, increased speed and payload – including the carrying of nuclear weapons) or brand new, cutting edge stealth designs.
Intercontinental Range is a given, with an absolute minimum range of transatlantic and back again, preferably on one tank of fuel. And the aircraft should be British, or at least possess British Engines and electronics, although if new, then feel free to make it up as you go along
Good luck, and happy flying