Battersea
Turbocharged
OK this is PURE speculation but.....
Under EU law a mfr MUST allow for a plus or minus 3% difference in tyre size as the law allows you to use tyres that have a circumference of plus or minus 3% against the original spec. Why? because even within a given tyre size there are differences between mfr's and tread depth makes a bigger difference than you might think.
You might be on to something there Chris, although I dont think it will be anything to do with the EU +/-3% malarkey. More likely to be the ECU working within a programmed tolerance between front and rear wheels. Going to a wide variation between the two is outside the range and the ABS throws a wobbly.
Using a 240/55 and 140/?? may have the same effect - who's going to try it first.