that sometimes cheap domain names don't always turn out to be as cheap as they seemed.
All domain name sellers must pay a registration fee for each name to the appropriate high-level registrar for a particular domain. In the case of the .co.uk domain level for instance the registration is handled by a company called Nominet UK and a company may only sell .co.uk names if they have been validated and certified by Nominet. The fees are standard, and all domain sellers have to pay them.
If a company then sells to you at a very low price it can only be making a tiny margin on the deal, and these companies will usually attempt to ensure that they can get some more income from you - often by adding a 'parking' fee to the registration cost, or by selling you a one-year registration when it should be two. In addition, some of these companies ensure that it's very difficult for you to move the domain name to a different name server should you want to host your domain somewhere else, or they charge a hefty fee for doing it. Another ploy is that they allow you to 'point' the name to a different name server rather than actually retagging it - thus ensuring that they'll be the ones to charge you the renewal fee when it falls due.
I'm not for a moment suggesting that all companies operate like this, but some do, and I'm advising you to tread carefully. Make sure you know exactly what you are getting for your money, and find out how easy it will be to move the name, should you want to do that. As long as you're happy then go ahead, but remember that £20 saved now could turn out to be a false economy later on.