You aren't conning your company unless you're misleading them, so if you want to be absolutely up-front you could let your boss know what you do during the flight and see if they complain.
I don't believe there's any strong standard of professionalism demanding that you to work while travelling. If there is I haven't heard about it, and I've had plenty of colleagues here in the UK who apparently haven't heard of it either. Many people do work while travelling for their own reasons: either because of deadlines (they would be working even if they weren't travelling) or because they figure they can get stuff out of the way during relatively "low-value" time (planes suck).
It may be that your colleague thinks all time is divided into "working hours" (must be used to maximise shareholder value) and "non-working hours" (used at your discretion). But when it comes to travel and overtime, especially unpaid overtime, that simply isn't the case. You've been asked to carry out a particular task (travel to the destination). You haven't been asked to do 12 hours solid work that happens to take place on a plane. Or at any rate you weren't asked clearly to do that, or you wouldn't be here!
A typical UK salaried contract that allows for unpaid overtime says that you work the hours required by your employer. I assume you're opted out of the working-time directive, which nearly every professional job in the UK asks you to do but can't require you to. So perhaps in theory a typical contract means they could ask you to work a 168-hour week. Your boss can certainly ask you to work while travelling, on exactly the same "unpaid overtime" basis that you were asked to travel in the first place.
But would your boss actually sit there with a straight face and ask you to pull that 12-hour shift in addition to the work at the destination? If not then it doesn't really matter what your colleague thinks: you aren't being asked to work the extra 12 hours and so there's no need to work the extra 12 hours. If you do your job on a "get the work done" basis then you're free as usual to organise your own time, and if you do your job on a "must be available to others during office hours" basis (for example because you're typically public-facing or have other duties that require a set schedule) then clearly that doesn't apply here since you're on a plane outside office hours and therefore cannot be available anyway!
Ultimately the difference between you and your colleague is, your colleague thinks he's been asked to do 12 hours (6 each way) more work than you think you've been asked to do. One of you is probably wrong (and I strongly suspect it's him), but if you're both right, OK, sometimes different people are asked to do different amounts of unpaid overtime. Sucks to be your colleague, but it's not a matter of professionalism.
Now, if the time you spend on the plane is being billed to a client, then you need to be clear what it is you ethically/professionally can and can't do with the time. For example, you might not be able to do work for a different client because that ultimately might lead to double-billed time. Also, the company can't mislead the client in terms of how the hours are itemised: they have a right to know what they're getting for their money, and whether you work on the plane or not might even affect their decision of whether it's worth paying for you to come to their site, as opposed to paying for you to stay home and do something else. But if that's the case you really should have said so in the question ;-)