Before deciding to make alternate arrangements for your retirement, you need to consider the benefit that your employer is also paying into the scheme (I doubt they will contribute to your mattress money).
Most employers match employee pension contributions, many as does mine, pays in 5% versus my 3%. Ignoring the tax benefit of paying into a pension (other tax efficient savings are available), based on an employer only matching your contribution; let’s say at retirement you have a pension pot of £100k. You will have only contributed to half of this amount, of which you can have half (£25k) tax free. Even if you were to decide to buy an annuity with the remained of the money, this would be based on a £75k investment, rather than £25k if only you had paid into the pension scheme.
The above is an illustration of the benefit of an employer scheme – of course it excludes the annual fee paid to the pension company – but you would pay this if you set up a private personal pension.