For one thing, you're looking at new cars when you should be looking at cars at least a couple of years old. Another thing is I would compare the rate of leasing the car without your employers deal compared to with them, as I know the british car market isn't anything like the US market(you guys pay ~2x what we pay on cars I think). Other things to consider are: do you need a car? I know the London Underground is pretty renowned as being a great municipal transport service, dunno where you live but you may not need a -car- that badly.
Another thing, is one of the takeaway's I got from the book "Millionaire Next Door" was that a lot of those who become millionaires may or may not have vices, but find ways of frugally having them. An example is there was one who enjoyed having new-er Mercedez. His solution? Pick up the 3 year old models at a steep discount, he get's his constantly improving Mercedez, and at a much cheaper price then anybody else, as the 3 year old ones had depreciated a lot of the way that they would depreciate, and after that the rate of depreciation slowed(and he was picking them usually off of the 3 year lease).