Make sure your PC isnt infected first, you probably got it bundled with norton antivirus protection racket ware as seems to be the norm, download the free 'Microsoft Security Essentials' you can find this on google, remove norton and install that instead. (having more than 1 anti virus product might sound like a good idea, its not and your PC would be horribly slow). Run a full scan after you first install it and perhaps schedule a scan for once every two weeks
Next up, if your using internet explorer to browse the web it would be highly recommended you change it for an alternative like Google Chrome or Firefox, IE is targetted by malware more often because people who use it are often less computer savvy and less security aware, so using it you are more likely to be infected by malware that could monitor you entering payment information
If you do stray from the well known brand sites then check things like
-Returns policy
-Company registration number
-Contact info (UK address and postcode)
-When you go into registration/login/payment sections of their site that SSL is enabled (https at the beginning of the address and a padlock shows)
If the place has a proper bricks and mortar UK address that you could potentially visit then they are within reach of the police and your less likely to get ripped off, if you are unsure use google streetview to get a look at their address.
My old man, who is a little savvy still got caught out buying sunglasses online as a gift. The site he visited appeared to be a geniune site and it had a .co.uk address but it didn't raise his suspicions so he didn't check for any of the above and for what its worth he didnt find the fake shades he was mailed from china to be all that bad but he couldn't give them as a gift
You won't run into any problems at all.
