I've heard the maximum BMI is 28 for the pre-employment medical but have never seen it written anywhere. The pre-employment medical is very similar but possibly very slightly more rigid than the annual JCAB medical. After the pre-employment medical you'll have your first JCAB medical prior to having the JCAB ATPL issued. This first medical is basically the same as the pre-employment medical with EEGs etc and the same sort of limits for BMI (again, nothing written that we can decipher). After that first JCAB medical things calm down a bit and they're not quite so exacting but, according to the rumour mill, they have a BMI limit of 30 before they'll report you to the JCAB and, allegedly, you'll find yourself on the treadmill for a 'stress' ECG test.
We're all overweight according to them but, to answer your question, 28 seems to be the absolute pre-employment limit, while 30 appears to be the 'employed' limit. Anything above 25 and you'll see red writing on the medical report.