The cinemas may be open but people just aren't going in any major numbers. Black Widow has made £13.8m in the UK in 3 weeks. Spider-Man Far From Home which released at the same time but in 2019 made £14.1m in its opening week. Obviously you would expect Spider-Man to do better but not by that much.
They may as well hold it off a bit and release it when the numbers have picked up. From the articles on it they seem to be suggesting it will get a cinema release, they're just not sure when. From the trailers it looks like the sort of film which should be seen in the cinema if possible.
Guessing your figures there are wrong if £300k is massive swing?