If you order a domain through Saffas or transfer the renewal rights, you will be assigned a standard Domain Profile which contains the domain contact information.
If this ever needs to be changed you can do so online quite easily ...
1 - login to the Billing Centre
2 - click on
'your domains' or 'domain names you hold'
3 - click the 'manage' icon alongside the domain to change
4 - click the link to 'contact information' to add the new details
The changes will take up to 48 hours to be reflected in the WhoIs database online.
UK Registrant Details
For UK domain names (co.uk, org.uk, me.uk) you can easily change all contact details online except the registrant name.
Unfortunately, Nominet do not make it easy to change the registrant name and this is a more involved process for which there is a small fee.