The last big contender that Merlin do not own is Blackpool Pleasure Beach...that is the only attraction on a Merlin Scale within the UK that is no owned by them! They would have a monopoly in Blackpool if they did...but it won't happen, they won't sell that gold mine, nor would Merlin wish to pay for it!
Amanda Thompson will never sell Pleasure Beach Blackpool, so Merlin don't really have much of an opportunity there.
In terms of attractions in the UK, I think Merlin are coming close to reaching their limit. Besides a 'punt' on either Lightwater Valley / Flamingo Land in order to establish more of a foothold up north, or possibly a bid for Oakwood (though I can't see that, as potential visitor figures are very limited due to it being in a difficult location to get to) there's no way they'll get another theme park in this country. But, honestly, they don't really need one. They already own the four most visited theme parks in this country - since PBB is an amusement park - and are essentially competing with themselves, hence why they're willing to make mediocre improvements such as Zufari and backwards Swarm, investing minimally in improving customer satisfaction.
Where I do expect Merlin to continue to grow is with regards to midway attractions. I expect to see Blackpool, Manchester, Birmingham, Edinburgh and London boasting the 'full set' of midways (Sealife, Tussauds, Legoland Discovery, Dungeons) within five years, with potentially more ideas coming along too. I think Windsor Castle is also a likely target, though I doubt they'll be able to convince the current owners to sell-up.