I think a number of UK parks have some degree of issues with ride availability at the moment. While from my personal experiences Thorpe Park seems to have suffered slightly less than Alton Towers (while the two were equal today, you can’t assess the situation conclusively based on one day alone), it still isn’t perfect, with the odd extended closure and breakdown here and there being par for the course most days. If anything, my one experience at Legoland probably had the best ride availability of any Merlin park I’ve visited this year; when I was there on 9th June, I didn’t notice any notable breakdowns or closures.
I don’t think it’s exclusive to Merlin either. I’m led to believe that Blackpool Pleasure Beach has had a number of things closed and breaking down, as has Drayton Manor, and even Paultons Park, while generally faring a lot better than most, has not gotten off completely unscathed, with extended downtime on their log flume and a couple of other smaller rides this year.
Could Brexit possibly be an issue for the UK parks? It must be harder for parks to get parts and things in to rectify extended closures and issues, which will surely make downtime worse.