I think the lowest 'qualifying' fare code is H. Fare explorer will give you Y for Economy (Flexible).

Try using http://www.travelselect.com/, specifying 'British Airways'. It will give you a selection of fares on your dates, and you can scroll down (or request more) until you get an H fare.

BTW, a MAN-LHR return in H is £177 plus taxes.

MAN-IOM comes in slightly cheaper at £168+.