Well, in the UK, you're only likely to register if you need to. Since most companies probably don't then they won't. If you're just holding peoples data for the ordinary running of your company, i.e. for accounts and records (invoicing) and do not share any information with third party companies, then there is probably no need. If you do share information or use it for any other means, then you must.
There's a handy walkthrough on the DPR's site: http://www.dpr.gov.uk/
(Under Publications>A Self Assessment Guide)
It can be a little bit confusing, but keep going back through the whole form and read everything.