Will 255/45ZR18 tire size fit Jaguar 2003 XJ sport without problems?

it wouldnt make sence to use a tire with the 45 ratio instead of the
40.  255 is the width of the tire in millimeters.  45 is the ratio of
tire height to width. ZR is the speed rating.  18 is the wheel size. 
a taller tire (45 instead of 40) would decrease handling ability. for
stock height tires look at

http://www.tirerack.com/tires/Compare1.jsp?startIndex=0&vehicleSearch=true&width=255%2F&ratio=40&diameter=18&sortCode=54150&minSpeedRating=V&ratio=40&width=255%2F&diameter=18&minLoad=S&search=true&pagelen=20&pagenum=1&pagemark=1&speed_rating=ZR&x=12&y=1

if your dead set on using those tires you may be able to find a
template at the manufacture's website

if you cant find a template you can make one. 45% of 255 is 114.75
therefore making the tire height 114.75 mm. make a 18 inch (wheel
size) plus 229.5mm (tire height times two) diameter circle and see if
it fits where the tire does at full  suspension compression.

or you can measure the distance between the tire and wheel well at
full compression and see if that is greater than the difference
between the new and old tire heights.