Thanks to all your suggestions ^ - really appreciated . Cunning plan refined to the following.

1. Do OLCI (does this mean On Line Check In??) and print off boarding passes at home. Presumably can select seats at this point too – Am I right in thinking I can do this 72hrs before departure?
2. Try ye-olde diversion tactics at airport to avoid cover being blown at luggage drop (may also include note in passport to ask attendant not to blow cover).
3. Nonchalantly go through Fast lane Security and if she asks why we can do this, say, ‘because Check-in said we could as it’s your birthday’. (long shot, but she may fall for it)
4. Visit MAN lounge but put it down to access via AMEX.
5. Land at LHR – traverse to the doors of CCR and then let her in on the surprise.

Also agree with those about telling her and sharing these moments (and if my cover is blown at any point I think I’ll just smile and hold my hands up!!) but if I can make it to the CCR before telling her, knowing my wife it’ll make the trip even more special.

Just a couple of further questions.

1. Re: spa treatment and cabana at LHR – our flight times give us about 3 hrs at LHR – is this enough time to have a meal in CCR and have spa treatment?.
2. Do I get boarding passes for JFK-LHR-MAN return at JFK? If I do, do I use a machine or check-in desk to get good ones to keep?
3. Thinking of taking cab for iconic commute in to New York, but would anyone offer any other suggestions?

Thanks.