When are you thinking of going? I imagine not for a number of months, so you at least have time mucking about with trying different packing techniques, both in getting the bike into the box (if you have not done this before with your disk Trucker), seeing what can fit into the bike box safely and still be within the weight limits.

"Safely" meaning what can go in without it being damaged, ie a sleeping bag wont get damaged, it will in fact cushion the frame a bit with bangs, same with a tent (usually poles are pretty secure and protected rolled up in the middle of the tent "sausage")
I always look at the bike in a box and visualize what is going to get skwershed given a given drop or whatever, and then place stuff to cushion the blows from directly hitting a given part of the bike (or stop one part of the bike dinging another part) using my stuff or bits of cardboard etc.

re your trip in Wales, if you are willing, I'd be interested in seeing your idea for a route, I have family in southern UK as well as in Wales (more in the north) and have toyed with the idea to fly into London, ride down to the south coast and then work my way along to Cardiff and up.

You dont mention a front rack, so does that mean you will be going with two rear panniers only, and stuff on top of the rear rack? My photo is with only two rear panniers chucked into the old red zippered bag we have, but as others have shown, you can get away with much lighter and cheaper bags to hold your panniers and such together, just throw on a whack of tape and for a flight it would work.