I don't tend to feel guilty about non-eco decisions I make, as I know we can all only do what we can do, and we can't be perfect all the time. I let others do as they please and don't evangelise.
Except, apparently, when it comes to your own husband who you seem unable to forgive or accept.
There is no other way to travel 2000 odd miles for 20 quid. None.
In his shoes I'd be totally happy to not fly, provided you paid for whatever form of transport we did take.
Also, while taking trains might be "an adventure" it also eats days out of your holiday, which may be something your DH doesn't want, given he doesn't see these people every couple of months. Your children will likely get very bored.
12 hours of train vs 2 hours of flight and 10 hours of relaxation/fun for a tenth of the cost seems like a no-brainer to me.
Also if you're driving then you're adding far more pollution to the air than if you fly because that plane was going anyway, and your car would otherwise have been sat at home turned off for a week or so, rather than burning 4000 miles worth of petrol.
By all means, plan to fly less than you have done previously. Likewise, eat less meat than previously and take more public transport than previously. But don't forbid flying, driving, or eating meat completely, unless both of you are equally on board with it.
I mean, how would you like it if he insisted on some environmental practice that you personally didn't want to do e.g. never having the heating on and just wearing jumpers and coats all the time?