jakeal has tagged me for this. "Under construction" in the UK means in the process of being built. So I am guessing that, as the hotel has been there for some years, you mean is the refurbishment complete. Personally, I have never stayed there, as it is not in an area of London that suits me. Nothing wrong with it you understand, just the wrong side of the park for me. I warn you, it is not close to Central London attractions.
I can't find much about the renovations except comments on Trip Advisor. It appears the work has been going on this month. See the posts and replies. London Marriott Hotel Regents Park - Top Reviews 2016
If I were you, I think I would ring the hotel to check what is going on.
Thanks for taking the time to answer Tommo781. I have stayed here many times...mainly because it is away from Central London . The trip Advisor posts were extremely helpful...thanks. I'm going to stay away for now.