Region locking is kind of understandable if a third-party home video company distributing the blu-ray only has a license from the rights holder of the title to sell it in a particular part of the world (and the rights holder licenses a different home video company to release/distribute it in another part of the world).
But in this case, BBC is distributing its own content to both the North American and UK markets. And as far as I can tell, all of their previous Doctor Who blu-ray releases for both of those markets have not been region-locked. The most recent example is "The Web of Fear," which BBC released on blu-ray in the UK just last month, but not (yet) in the US. And the UK blu-ray is indeed playable in all regions.
So, based on that track record, the UK "Galaxy 4" blu-ray might turn out to not actually be locked to Region B, but actually be all-region just like all the previous UK Doctor Who blu-ray releases. We haven't actually seen the back of the packaging, where the region coding would normally be listed (if applicable). Currently it is listed for pre-order on Amazon UK, HMV, and Zavvi. Amazon UK and HMV don't specify region coding for this item; only Zavvi says that it is Region B. And it looks like Zavvi lists all UK releases as Region B, just as a standard for their intended purchasing audience. I notice that they inaccurately list previous Doctor Who blu-ray releases as Region B even though those releases are actually not locked to Region B.
TL;DR, it's likely that the UK "Galaxy 4" blu-ray won't actually be locked to Region B. Let's wait a month or so for more detail, including a look at the back of the package.
Edit: After looking up more of the UK releases, I found that some of them are indeed locked to Region B.
Kor