It all comes down to what your expectation of the role of the armed forces is for the UK in the 21st century.
Aircraft carriers are all about force projection - a potent symbol of a countries ability to conduct military operations a long way from its own territory. Undoubtedly useful if your role is to be the worlds policeman, and of some benefit in protecting UK vested interests outside the immediate territorial borders of the UK, but of little benefit otherwise - and especially if you have an aircraft carrier but no aircraft to fly from it. And wasn't it the case that they decided it would be cheaper to build 2 and then immediately mothball one anyway, or sell it or something?
The expenditure on aircraft carriers, rather like our spend on the Trident Missile Defence system, is largely symbolic, allowing us to keep our permanent position on the UN security council - to "punch above our weight", as it were. But when you factor in the cost to the country of such military spending, you have to question whether they are truly of value to the british taxpayer. After all, at a combined cost total (Trident and aircraft carriers) of around £110 billion or so over the next 10-20 years, that's a lot of money that might be better spent improving travel infrastructure, or improving schools and hospitals, or any amount of other domestic projects - Even paying down the debt! :)