A domain name on the Internet is the equivalent of an electronic address for your website. Domain names are recorded on a register e.g. by Nominet which is separate from the Trade Marks Register on which trade marks are registered.
It is possible to object to another person recording a domain name similar to your trade marks but this is significantly easier if you have a registered trade mark.
Conversely, it is possible to object to the registration of a trade mark relying upon your domain name but again it is significantly easier if you also have a registered trade mark.