I know there's been a policy change in regards to UK shows and the way cast are credited... But it's worded in such a way that doesn't draw attention to the different conventions as regards to how UK shows approach title credits vs. end credits. Putting an actor in the opening credits of UK shows does not automatically denote them as 'main cast', which seems to already be causing confusion as to how things are listed.
For example: http://www.tvmaze.com/episodes/1014738/midsomer-murders-19x03-last-man-out/castappearances, tonight's episode of Midsomer Murders... Yes, technically all those people are in the opening section of credits, but only Hughes, Dudgeon and Hendrix are considered 'main cast' (if you watch the title credits, they're actually subtly sectioned into 2 groups).
Although some of the cast listed in the title credits are recurring (Dolman and Virk), the rest are single episode cast. But UK convention dictates they go in the opening credits because they're the main players in that particular episode, they're not 'main cast' as TVMaze would define that role. For shows like Midsomer (and off the top of my head, Death in Paradise), this would lead to hundreds of characters being listed as 'main cast', which is just wildly inaccurate. For this example in particular, ONLY Hughes, Dudgeon and Hendrix are considered the 'main cast', all the others belong in guest cast.
So, although I understand what the new change policy is trying to convey, it's not conveying it all that well, especially for people who aren't familiar with difference in UK title/credit conventions and their differences from say, North American conventions. You can't just say all main cast are listed in opening titles, because guest cast deemed particularly important to each individual episode are often listed there as well.