I'd disagree on the selection and use of the end or cross grain cut for your sign. It provides the most striking texture and unusual "target" motif. I've used the same cut here in the U.S. for years for multiple purposes, especially TB's shunned turned bowls.
Having said that, the technique of air drying the piece before work begins is time consuming and somewhat tedious. My own experience, especially with cedar (which is notorius for cracking) involves a good moisture meter for testing as well as a location that I can store the cut so I'm not looking at it every day, anxious to get started. I start it an unheated shed and move it to a heated workshop.
A good air dried blank, such as yours (I too admire the quality of workmanship) may take as long as six months.
There is, however, an excellent product here in the U.S. (and, I suspect the U.K.) called Pentacryl. It's applied as per the directions and has worked well for me, including pieces that had begun a crack. I especailly like the ability to use any finish after application.
One thing I would recommend... the large "knot" (where a branch had grown out of the mother log) should be stabilized. I really like working with copper and bronze and have several pieces of "strap" from both materials. I would carefully, on the reverse side, of course, chisel out a shallow channel across the joint... extending at least 3 inches into the branch as well as the main log (even longer if you can) probably about half an inch wide to accept a strap and using copper or brass screws (the heads of brass and copper screws often twist off in hardwoods... simply drive the same size of steel screw and then back it out and use the copper or brass, countersunk, of course) as well as a good multi-use glue, imbed the strap. (Actually, due to the size of the knotted area I'd use maybe 2 or 3 such straps.) This will certainly stabilize that area which will be a problem, left untouched, sometime in the future.
I'd use copper or brass for the effect of the patina over the decades, even though it's on the reverse. Imbedded, it won't interfere with hanging the piece. (For effect, I'd personally consider doing the above on the face of your sign).
Lastly, the blank is cracking due to loss of moisture... to attach a full cut piece of plywood or any other such assist, will, in my opinion and experience, only cause the blank to "bow", since it can't continue to shrink normally. But, that's my opinion only... woodworkers are well known for having differences of opinion...