Could that be because it's a carrier branded release, so they will need to take an earlier snapshot to perform their testing, and add their apps, etc? Will be interesting to see what the unbranded BTU release is based on.
I don't believe O2 bother to actually customise anything except the boot screen, but they do typically take AGES to perform testing. Quite surprised this version is so recent, they were always several months old on the Note 3.
Hopefully BTU will be newer though and I will probably then switch to that as its kinda important to have the latest Samsung fixes with having the GearVR.