So for further reading on this if anyone is interested (long post incoming)
Went to Repmart in Akihabara, opposite Echigoya - lovely shop and one thing that is different is that this one has a lot of space, it doesn't feel like a carboot sale.
They had an in store special on the SD6 which was too tempting not to at £350ish. Also picked up a decent armourer's wrench whilst I was in there.
Then moved to Kyoto, and popped into Gun Shop Gurka in Osaka who deal with 2nd hand stuff. TM MK46 marked as B grade, this thing looked as good as new, bar some light scratches on the stock, and no outer box. Bargain at around £500.
Also in another shop down the road, elusive TM MP5 mags at £20 each, so picked up 4 of them, before the wife screamed at me.
So this is when it gets interesting.
My flights were ITM-HND with JAL, then HND-LHR with BA, all on the same ticket but handled by the respective airlines. I had spoken to BA ahead of time and they said if JAL check it in at ITM, then all gravy.
I had this confirmed in an live chat and had the print outs. I also phoned JAL in the UK ahead of time, and was told to email them the fact sheets and my flight details. They got back to me and said all good, they will put a marker on my ticket to have it inspected before being checked in.
In preparation, I bought 2 large cardboard boxes and taped them together into one long one (still within airline limits) and bubble wrapped it all inside.
Got to itami, checked in the suitcases no problem. Inspected the guns and a police officer walks over, gives it a quick look and a thumbs up. This is going too good... Then JAL bring out 2 huge metal flight cases. They want to ship the rifs in their special weapon cases. Ok not a deal breaker, I can just grab them at LHR. No dice, they are for domestic flights only, meaning I would need to pick it up at HND and re-check it in. Urghhhhhh.
So I ask "how do i get these back to London once they are out of the metal cases?" They then check in the empty 6ft cardboard box as well.
Get to HND, pick up cases and box, transfer them back to cardboard box an then lug it across to the international terminal to check back it.
Approach the desk where it's Japanese BA staff. Explain to them what has transpired and hand over the spec sheets of the 2 which i had printed out in advance. The nice lady nods, then talks to an older gentleman who looks at he spec sheet - says something in Japanese but i pick out the words Marui, and Airsoft. So they take me over to oversized baggage, and run it through an xray, no need to unbox this time. Nervous wait and then a thumbs up. I break out the roll of gaffer tape and seal down all the openings 3 times over.
Get to LHR, box pops out, border patrol/HMRC are all on strike and i walk through the green channel with no issues and from touchdown on the tarmac to my driveway in 90 minutes.
Overall if I were to do it again, i would by a softcase/hard case and bung them in that if it weren't a mk46.