Englishal, you can't run a car diesel engine solely on Jet, but you could (albeit ilegally) add about 40% jet to diesel in summer & 30% in winter. However it wouldn't do your engine much good, the lack of lubricity in jet would cause excess wear, and Kero produces much less power in a diesel engine than derv.
It's also incorrect to think that customs couldn't catch you, some jet in the UK is from DPK (dual purpose Kero) which contains coumarin - an 'invisible' customs marker, easily detected by analysis.