I've just had another look at the pic you provided of your rusty jacking point and marked in red where the floorpan has rotted away. Also marked is where the inner sill has rotted in yellow.
There are other bits on the floorpan closer to where the fuel tank is that are questionable but tricky for me to be certain whether it's surface rust or gone through but they are the darker spots you can see near those brake lines in the bottom left of the pic.
Not only will you be completely snookered if you get a flat tyre while out and about as that jacking point won't lift the car but the important bit is that the corrosion is within 30cm of a seat belt mount - in other words, an MoT failure point.
To give you a bit more of an idea whaat's happening, mine rotted in the same place, both sides. I only took pics of the drivers side but you'll get the idea :
Doesn't look too bad there but here are a few more once i'd cut the roten stuff away :
The outer sill is completely cut away there and you can see the remains of the inner sill hanging down into the hole. Jaguar used several pieces to create the inner sill, alternately layering them and spot welding each piece to the next. I don't know if this was done to create a break-point for corrosion but it certainly worked as that! I used a similar idea but seam welded each layer so i wouldn't fall foul of the MoT regs, not that it could be seen easily with the outer sill repair piece in place but you get the idea.
First inner sill repair piece in place, welds ground down and weld through primer applied, ready for the next piece. Now it's all finished and has a new MoT, i can certainly tell you it's as solid as, if not more solid than, when it left the factory. Despite the obvious curvy nature of the bodywork in general, there are no complicated compound curves except a very slight one at the rear end that can be pushed into place (even with 1.2mm sheet) to be welded then trimmed to suit after with the disc cutter.
I wouldn't call myself an expert welder by any stretch of the imagination but was quite chuffed when i asked the tester "What's the verdict?" and the reply came "Yeah, that'll pass - nice job!".
Like everything with bodywork, preparation is the bulk of the time, the actual welding took about 10% of the overall time i spent on it, most was prep work, making/shaping repair patches/panels. All in, i probably spent about 12-15 hours doing it all and out of that, about 1-1.5 hours was actual welding.
Your big decision is whether to do it yourself, get a mate to do it at mates rates/mutual exchange of labour or put it in to a garage that can do it or a bodyshop. On a DIY basis, about £250 including buying a cheap gasless MIG, metal and consumables (wire, grinding/cutting discs etc) or likely to be £2500 at a bodyshop and the rest, somewhere in between.
If you're keeping it, whatever the cost is shouldn't be an obstacle, if you're selling it then get it done as a buyer would likely offer scrap value if they saw the current state of your sills, not to mention calling into question if the MoT tester was Mr Magoo! Once done, if you're selling, get another MoT done as i wouldn't feel confident buying a car that had been passed with that level of corrosion and an alleged 12 month ticket!
Probably not all of what you wanted to hear but i think others will agree that nobody wants to see you driving around in an unsafe car!