If that is after paying your taxes then you are doing alright.
Exactly, after taxes.
Let's take the lowest value of
£400 a week, after putting the tax aside would it be like
£320 or so??
Then expenses, let's say:
£50 food, various bills (Council Tax, mobile phones work and personal, broadband, HEATING!

, software subscriptions etc) - shall we say
£100 for bills?
Then work expenses - advertising, equipment and clothes (depending on your niche and if you've been working for a long time and stayed the same size throughout probably v little would need to go on clothes) - I don't know, maybe
£30?
Hairdresser and beauty treatments that you need for work. (I was paying £100 a month for a hairdresser in London - cut and root colour, nothing fancy.) Let's say,
£30 a week?
It totals to £210 so far. Which leaves you now with
£110.
Then an Oyster card or other travel expenses.
This is when you have no dependents (kids, pets) or debts. Or need a weekly session with a therapist.
And then, let's imagine that after 10 months of moving out of the property, an energy company slaps you with a bill they pull from their behind. Because why not.

Or whatever life decides to throw at you.
I don't think you need to have "horrendous" expenses to feel that with £400 a week you're on a tight budget.