I have been looking into the same idea for the last few years. Im also in the UK and if you know anything about kit car or specials then you will have heard about the SVA test.

The main problem with an XJS V12 is it has a wheel base of 101 inches, all Batmobiles (including jay's) will require a wheel base of 141 inch (give or take an inch, look at the blue prints in the image database) I know all this because I spent a wet afternoon in a scrap yard measuring an XJS.

The other problem you have is the fact that the XJS is what is called a single body construction. This means the body chassis is all fabricated and welded in one go (unlike say a land rover which has a chassis with a separate body bolted to it) This means to stretch your xjs to 141 inchs you will have to cut the body into two and manufacture a new centre section and weld it all back in, then do the same with the prop shaft, fuel lines, brakes etc.

In doing this you will have spent a lot of money and because you have modified your XJS it will now require an SVA. A Batmobile replica will probably not pass an SVA test the main problems being headlight location, indicator position, lack of a rear window, the split windscreen is illegal and it requires wing mirrors. These are the most obvious problems that I have found from my research. So on the whole trying to road register an 89 replica in the uk is a bit of a mare.

Now for my build idea. I have been researching this for the last 2 years and the only thing that stopped me from starting this was a motorcycle accident. The car I will use is a reliant scimitar (99 inch wheel base separate chassis with a glass fibre body bolted to it). The batmobile body I am going to build from scratch scaling from a hotwheels model, based on the 99inch wheelbase but because I don't have to modify the chassis no SVA test is required!

If you want to talk further about this please email me.