According to the last comment in Autocar it is Slovakia.
"One question hanging over the new Defender is where it will be produced. It is unlikely to be built in Britain because the two JLR factories that make aluminium-bodied cars, at Solihull and Castle Bromwich, are close to capacity.
That leaves the firm’s new plant in Slovakia (where the Discovery, to which the new Defender will be closely related, will be built) as the most likely location. However, outsourcing its manufacture is another option, with Magna in Austria set to build an as yet unconfirmed second JLR model in addition to the Jaguar E-Pace."
However if figures are down on those models then it would be good to keep it here. I guess the Brexit tariff issue will be the decider. A good chunk of the industry is pausing to see what happens?