It is not very useful to compare them with flu because people are much more likely to have flu than to have covid. These rates seem very small but remember that the study started in the beginning of february, when there were likely only a handful if any covid carriers in the uk.
If you could preselect a group of people that you know had covid and measure their death rate, the numbers are likely to be much higher. This study did not do this. This is probably because this would require that all patients are tested for covid as they visit their doctor and this was obviously not happening.
Also flu deaths have been greatly exaggerated. Which explains how with a supposed 50k flu deaths in a year in the USA, it has never inundated hospitals like this.
85k coronavirus deaths haven't inundated hospitals in the US either, except in the absolute epicenter in NYC, and even then not to the point originally feared. It's very easy to imagine 50k flu deaths geographically spread evenly not overwhelming US hospitals, especially given that they'd be spread over something like 3x the current coronavirus epidemic timespan. I'm very skeptical of the sudden "oh those flu deaths aren't real" narrative.
If you could preselect a group of people that you know had covid and measure their death rate, the numbers are likely to be much higher. This study did not do this. This is probably because this would require that all patients are tested for covid as they visit their doctor and this was obviously not happening.