I truly believe that if Smiler opened tomorrow, it would have the longest line.
I also think it's fair to assume all UK parks are suffering reduced attendance - I really don't think the kind of people who'd be put off going are smart enough to figure out which parks are Merlin and which are not.
I would guess that Legoland is least affected, actually. Don't get me wrong, I'd rather walk barefoot over a mile of awkward lying Lego bricks than go to Legoland, but for some unknown reason, the public love the place. No amount gimmicky fairground rides with capacities lower than a Twizy, stretches of barren unthemed nothingness, broken animatronics and dirty figurines can sustain the public appetite for that place. My logic is that Lego's branding and distance (not literally... I mean like, brand wise) from the rest of the Merlin parks, and actually wider UK industry as a whole, would keep it safe. It's not "one of them" funfair parks.