Many advisors say that you need about £250,000 to retire comfortably. But the reality is that most people in the UK retire at about age 65 and they on average have a net worth of £150,000. In fact that figure is actually skewed as the mulit millionaires make that figure higher than the reality.
To find out how much the averag Brit retires on, you should find the median. The median in the UK for the net worth of the average retired 65 year old is more around the £100,000 mark.
I would personally say that if you have a net worth of around £200,000 to retire on in your mid 60's, then that is more than enough to have an enjoyable life. I know several people who have retired on a networth in this region and they are more than comfortable. Most of the £200,000 should be invested though, with the rest used for day to day essentials and enjoyment
So I would say that .£200,000 would allow you to retire comfortably at age 65 in the UK. But if you want a very comfortable life, then a net worth of £400,000 would give you an extremely comfortable life. Where you could go on holidays frequently and go out most days of the week without having to worry about money too much. It would allow you plenty of other luxuries as well.
Note that this all applies to most places in the UK that are outside of London. If you live within London, then I'd say that you should retire on a net worth of £300,000 for a comfortable life and a net worth of £500,000 for a very comfortable life within London.
Note, don't feel too bad if you don't think that you can't retire on these net worths, because about 60 to 70 percent of retired Brits retire on net worths less than the figures that I have been given. I hope that my answer was helpful to your question.