Nominet allow UK Residents (not business) to Opt-Out of WHOIS.
This means that should anyone do a WHOIS search on your domain then very little information is displayed.
This is a Nominet feature and not a feature afforded automaitcally to other registra's.
.com, .net, .org - The Terms & Conditions for these domains state that WHOIS information must be regularly updated so as the information contained is alwyas correct.
There is no opt-out option for these kind of domains.
There are some companies who offer "WHOIS Privacy".
You must remember that this is not an offical service and that the T&C's for the domains does state that WHO information must correctly reflect the owners details.
The "WHOIS Privacy" service will usually replace your details in WHOIS with those of the company who are looking after your domain name.
This means that any WHOIS lookup will fail to display your details, instead displaying the hosting companies details.
For the most part this is OK, nothing will really happen.
If anyone reports your domain as having incorrect contact details, ICAAN can get involved and request the correct information is entered.
Failure to do so and ICAAN can take the domain away from you.
Also as using "false information" in WHOIS is against T&C's, in theory ICAAN can just take the domain away without warning - again, you break terms they take back the domain you are "renting" from them.
To be 100% safe, just enter all of your correct details.
To be probably quite safe use "WHOIS Privacy" from your hosting company as ICAAN will contact them before simply taking back a domain and you will be able to correct things.
Under no circumstances just enter false details - if its reported to ICAAN and for example the contact details are totally fake so they can't get in touch with you there is a high chance you'll simply lose the domain.