I was about to post, thinking we were talking MAN T3, but based on the OP's location I'm assuming we're actually talking London.

But in case anyone's as daft as me (or just wants random information dished out with a smile), I used Fast Track at MAN T3 a fortnight ago. I was travelling CE from LGW, but the MAN-LGW leg was Domestic. The guy at the front of Fast Track just waved me through without looking at my boarding pass, and the lady who checked and scanned it had no problem letting me through for just a Domestic. Now, I was legitimate for Fast Track, but I'm not sure the CE leg was confirmed at any point - which could imply, in the case of MAN at least, the first person is the one you need to distract :-)