Thanks for the input everyone..
So, looked at a couple of hard cases in person...

The one like Angry Bear suggested is great, and obviously a dedicated rifle case...
But its 53 inches long, not very deep, and very cumbersome..
My trip is complicated by 4x train journeys, and it just doesn't feel like the right tool for the job.

As someone said, this will be a "One off" situation, and the rifles will never be in this case again.
So, I found a solution... I think..
I bought a used Pelican 1605 AIR hard case.
Like this...
https://www.pelican.com/us/en/produc...16050-0011-110

Its about 29" X 17" x 10" or 736mm x 430mm x 250mm.
Close to a normal suitcase size.
Paid $125 for it, and I think it will work good.
The foam in it is already cut to whatever i had been used for before, so that will be coming out anyway.
Stocks should be able to fit in too, either straight or on the diagonal a bit.
I can then use it for Camera gear later.

I spoke to VIRGIN, and the guy told me the 65 GBP charge is a normal extra bag charge... so if I go on a ticket that includes 2x bags, its free.
Spoke to ATF and the very nice lady looked up the Air Rifles in the ATF Database... which I thought was COOL...
She declared there are no restrictions for import, so that was good.