Keeping the car outside will be fine.
I'm guessing you will not be working on it yourself, as you are asking the question.
The trouble is both cars are approaching 20 years old, and maybe 1/4 million miles. The Jag will cost as much as a new one to run and repair, and they are becoming rare.
The GTO is much more rare, and is loaded with gizmo's controlling the motor, 4wd, 4ws.
Both could end up costing more than they are worth to keep on the road.
Personally I would buy a nice V8 S-type for silly money and run it into the ground.
To the GTO I much prefer the Nissan Skyline, having owned three. They are bulletproof. The engines sound glorious and the turbo's are blisteringly quick. My favourite was a 2.0, you could ring it up to 7,000 rpm. The trouble is they go for silly money and the boy racers modify them.
Here is what the RAC have to say about the GTO.......