How I foresee the British isles developing, in the long term (ie many years), in order of most likely to least likely addition, based on the comment about "lots of content".
-Anglo-Saxons/Anglia in Antiquity is virtually certain to form an alternate start path to the Romans.
-Ireland or Scotland, maybe Wales in Exploration: the next most likely, and still very likely, as an alternate path to a united Great Britain via the Celtic world. 85% likely to happen. If we get only one I think Scotland has the slight edge but not much on Ireland and Wales is well behind, but I wouldn't sleep on the possibility we get two, in which case Wales actually seems very likely to take the second slot to me, between the chance for dragon banners, longbows and arthurian mythology references.
-Insular Celtic antiquity as a precursor to the one (or two) civs above will probably happen as well. My personal favorite there is Dal Riata, but it could be the Picts, the Iceni, the Brythons, or half a dozen other options. This is more of a long term addition.
-Tudor England probably waits until all of the above are done, and even then may be limited to only adding Elizabeth (or Francis Drake, or Walsingham, or Old Billy S.?) as a leader. And as I previously mentioned, I think we don't see Tudor England until we're up to a lot of civ in every era, like 40 or 50/era.
-A modern celtic inclusion appears very unlikely, since Ireland only appears very late in the modern era and the whole point of Great Britain is that it pretty much united everything else (and Ireland).
Taking a wild guess of one British civ per release cycle (with each release cycle being about a year), I'd say we're looking at Release Cycle I (pre-first expansion DLCs) having Great Britain, of course ; Release Cycle 2 (Rise and Fall) adding the Anglo-Saxons, Release Cycle 3 (Gatherin Storm) adding Exploration Celts, Release Cycle 4 (NFP) or 5 (Leader Pass) adding Antiquity Celts as well, and Tudor England if it happens being held back for a still later Release Cycle 6 or later.
I'm not speculating about any fourth age additions just yet.