I did something very similar a few years ago and it worked out just fine.

To avoid her finding out at the check-in desk in MAN, you could casually ask her to wait and look after the hand baggage a short distance away whilst you check the suitcases in. She doesn't need to stand far away, just enough so she can't spot the tags being put on the bags or hear the agent mentioning the "F" word but close enough that you can point her out to the agent when she checks your passports. This trick is easiest if there is a reasonable queue but a bit more suspicious if there isn't.

If you hang on the JFK boarding passes and find a way to convince her the lounge access is normal then it should be easy to keep it secret until you get to LHR. If she is not a flying geek she won't even notice you using any fast track queues.

Good luck with it all