On the free, ad-supported live tv channels and the on-demand video library, Plex Pass does not allow you to access additional media items, and it does not remove the advertising.
As explained above, Plex Pass has zero influence over the availability of streaming content. This is all free and ad-supported.
The geo restrictions on that content don’t change, whether you have Plex Pass or not.
i.e. you can already see all what’s available to you in the UK even now, without Plex Pass.
Not really. It allows access to the “Live TV & DVR” feature, which doesn’t provide any media content on its own. All it does is to allow you to use a digital tuner device to bring over-the-air live TV that is available in your own area into Plex.
(That tuner device is not supplied by Plex. You must obtain it yourself and integrate it into your server/home network.)