Yes.Is your calculation based on only withdrawing the potential growth (eg 3.5%) and keeping the £350K invested?
You could take more and reduce the £350k to zero over a 25 year period for example.
My calculation is based on keeping the £350k intact and living off the interest indefinitely.
You could of course retire early, ensuring you had about £15k in savings for every year before your state pension kicks in.
£100,000 under the mattress means you could retire at 60 and live on £15k a year until you were 66 at which point you would get about the same amount from the state pension.