Flock of Gulls
I'll take the £20m and deliver a Flock of Gulls (no, not the prog rock band) trained to associate the top of drones with food using the magic of Pavlovian conditioning.
Gulls, Herring, Anti-Drone, Number 1, Mark 1.
My logic is undeniable; they're smart enough to train, dumb enough to condition. There's bloody faaasands of 'em just knocking about round RN facilities. They're aggressive buggers, and surprisingly large. I only need to train the first generation, and maintain their interest by occasionally sending up a food drone to keep up the pretence.
Then I can sub-licence to BAe, who can then charge the gubbermint a bazillion pounds to upgrade to Mark 2, which will be thirty two years late, 450% of budget, and be a confused otter strapped to the underside of a crow.