For LHR-GLA the way to figure out is to look at the seatmap and check if the middle seats are there or not. I used Expertflyer and for that flight (BA 1482 on September 27th) only rows 1-5 are Club Europe. 10 DEF are occupied

The divider between Club and Y may move, but be sure you will be moved as well in case your seat has moved to another cabin.
Note that BA222 on September 26th seems to be operated by a 787-9, not a 787-10. Rows 16-21 are PE on that type. Of course it would be great if you'd end up in PE for the Y miles!
BTW: my (now deleted) comment about domestic UK only having Y seats was no longer correct. It used to be the case when I lived there.