Re: Of all time?
If you want to count none UK defence procurement disasters, you can't look past the Australian navy Seasprite project.
Order new patrol boat in conjunction with Malaysia
Realise you can't fly exisitng very capable helicopters off these new boats
Flail around for a replacement and decide to refurb some 2nd hand US seasprites
Malaysia pulls out, so the patrol boat gets cancelled
Keep the Seasprites for inexplicable reasons
Spend the next decade and 1.4bn AUD trying to get them flying
Eventually realise they will never meet safety standards and scrap the whole project.