Your gut feeling is absolutely spot on - you shouldn't be worrying about pension now, not at the age of 25. Assuming that you're not a footballer in the middle of the most productive part of your career and already have a fat wad of crunchy banknotes under your pillow that you're looking to set aside for a rainy day when you won't be able to play at your prime any longer. That doesn't mean you shouldn't invest, nor that means that you mustn't save.
There are several factors at play here.
First of all as a young person you are likely to have a high tolerance for risk, there is still plenty of time to recover should expected returns not materialise. Even a pension fund with the most aggressive risk / return strategy might just not quite do it for you. You could invest into education instead, improve health, obtain a profitable skill, create social capital by building connections, pay for experience, buy a house, start a family or even a business.
Next, as a young professional you're unlikely to have reached your full earning potential yet and due to the law of diminishing marginal utility a hundred pounds per month now have greater utility (i.e. positive impact on your lifestyle) than a seven hundred pounds will in 7-10 years time once your earnings plateaued. That is to say it's easier to save £700 month from £3000 and maintain a reasonable level of personal comfort than carve £100 from £1300 monthly income.
And last, but not the least, lets face it from a human point of view - forty years is a very long investment horizon and many things might and will change. One of the downsides of UK pensions is that you have very little control over the money until you reach a certain age.
Tactically I suggest saving up to build a cushion consisting of cash or near cash assets; the size of the stash should be such that it is enough to cover all of your expenses from a minimum of 2 months to a maximum of a year. The exact size will depend on your personal comfort level, whatever social net you have (parents, wife, partner) and how hard it will be to find a new source of income should the current cease to produce cash.
On a strategic level you can start looking into investing any surplus cash into the foundation of what will bring joy and happiness into the next 40 years of your life. Your or your partners training and education is one of the most sensible choices whilst you're young. Starting a family is another one. Both might help you reach you full earning potential much quicker. Finding what you love to do and learning how to do it really well - cash can accelerate this process bringing you quicker there you want to be.
If you were a start-up business in front of a huge uncaptured market would you rather use cash to pay dividends or finance growth?