Goldberg said:
If anyone has any suggestions here, that would be most welcome. Ideally, I want something that holds a lot and disguises the smell.
My first advice would be, whichever high-performance diaper you end up picking, wear it with a booster or two. That makes a lot of difference.
ABU cushies are more cute than absorbent, and the single-tape design might not be the most efficient at preventing leaks. If you want to stick to ABU you should go with their top of the line products, which from my experience would be up to the task. Rearz would perform similarly to that, so another good option. Dry 24/7s are another good brand to go with as they have excellent wicking so they are able to distribute the wetness throughout all the padding better than ABU and Rearz, even though they have a very much different fit, and over here in Europe they cost a lung and a kidney.
Since you're from the U.K., and specifically asked for something easily available there, my advice would be Betterdrys, which are in my experience more performant in real life than all of the aforementioned brands and can be purchased for relatively cheap. A boosted or doubleboosted Betterdry will keep you safe until you reach your destination, of course it'll be quite poofy by the time your flight is over as they swell up significantly when wet, but if you want a diaper that holds for 10+ hours discreetness needs not to be a concern. Wearing a onesie to give him some support from sagging would be a good idea too.
Some people have mentioned body scanners, in my experience I went through them with soaked padding before and the only wetness the machine was revealing was the sweat on my t-shirt from carrying a backpack. Even in the case you got questioned about your diaper, be straightforward about it and do not worry, because it's not illegal to wear and use them!