Cash + Mattress = Pension.
NO!
This a terrible idea, not only is your money becoming less in value (inflation) you are also engaging in a loss due to opportunity cost (risk free rate, generally interest).
Not only that but the money you are keeping is net income whereas all pension contributions (up to a point) are taken from gross income.
I would definitely check to see if your employer offers an employee pension scheme as these generally better options than any personal investment you make. DB (defined benefit) schemes are generally a good deal (especially for more senior employees) and as the name implies your retirement benefit is calculated using a set formula, you pay a certain amount each interval and come retirement time your employer pays the difference between what you've accumulated and what the calculated benefit should be. The expected present value formula for this benefit is generally something like
n/R * Salary * avg_projected_salary_factor * annuity_factor_for_benefits * probability_of_retirement_at_ages_between_now_and_NRA * discounting_factor
where n is the number of years in service and R is the accrual rate. This is a simple example and real benefits are generally far more complicated.
Another option may be a company Defined Contributions (DC) scheme and in these schemes both you and your employer put a set amount of money each (lets say) every month or so and come retirement you would take the money in the pot and purchase and annuity from an insurance company. These are more flexible and may be a better deal for you if you are a younger worker. In general these do pay out less than DB schemes which is one of the reasons the latter is gradually getting phased out.
In both cases you may take up to 25% of the money in the pot as a tax free lump sum upon retirement.
If your employer doesn't have a company scheme for you to join you can always invest in a Self Invested Personal Pension (SIPPs) [you select an area you want funds invested in and a fund manager will invest your money into a portfolio of assets from that sector, performance is determined by the knowledge and understanding of your fund manager.] or a Stakeholder pension (these are getting phased out too in favour of personal accounts.) The downsides to these is that there is no employer contribution and you pay the administration charges yourself.
Many pension schemes also include ill health retirement benefits, benefits for death in and post service for spouses/partners and dependants.
So in my opinion paying towards a pension is definitely a worthwhile investment. Main reasons being: tax free contributions (to a point), fund growth (LPI factor or otherwise), employer contributions of some description (assuming company scheme), ill health/death benefits, up to 25% tax free lump sum upon retirement (you often have the option to commute this into a larger annuity)