Do you need to register with the Information Comissioners Office? If you're just holding the basic name and address information the chances are you probably don't. But it takes very little to put you over the line.
When we do a contact form on a website it is generally just things like your name, address etc. The basic information that is needed to be able to get in touch with the enquirer. That information is held on a database within the website data. It can be downloaded and used for other things, maybe sending out a news letter. That is where things get a little bit more complicated. If you use a mailing list manager such as MailChimp you are now processing the information. You will have information about whether they saw your newsletter or not for instance. That could well put you over the line.
I have quite a few concerns about the way some organisations manage ones' data so I think ICO is a good thing. Most companies and concerns are very conscious about the data the hold on you and take great care not to abuse it. Sadly that doesn't apply to all so a body such as ICO is essential.
If you need to find out whether you need to register use this self assessment tool.
As a web developers we will always try and advise you when there may be a legal issue involved with something on your website. But as we aren't lawyers we can only point you in the right direction. Ultimately you need to check the legal aspects for yourselves. InBrief.co.uk do provide some useful guidance in all things legal.