80%+ at peace time. I.e. not with mandated farming practices and not with a national focus as part of priority to fix this.

Unlike the Sudan we don't produce the remaining 15%+/-5 because we're a wealthy nation and can engage and have become semi-reliant a little on trade. And our tastes have changed as a result of the availability of luxury goods. This is wildly different to not having capacity or infrastructure which can be tweaked. (I'm not saying magically tonnes of milk and honey appear from nowhere) And this is ignoring the scary word of rationing.

Sudan either can't, won't or is constrained from producing the missing part of the food required to feed their people. But that's more linked to US foreign policy than this hypothetical discussion otherwise.

Edit in not even touching RoI vs UK army. I'm not pretending any show down would be bloodless, but the UK has a few plain numerical overwhelming superiorities. Again stating simple facts.