For means-tested benefits, it works like this.
If you have less than £6000, it does not affect your benefits.
If you have over £16000, you do not get any means-tested benefit.
If you have between £6000 and £16000, you will be deemed to have a notional income from it and this will be deducted from your benefit.
If you are of working age, this figure is £1/week for every £250. If you are of pensionable age, this figure is £1/week for every £500. If you are between 60 and 65, someone else will have to tell you whether it's £250 or £500 that applies, because I've forgotten.
You will see that the notional income bears no relation whatsoever to what income you could actually derive from investing your money.