To determine the classes under which your trade mark should be registered, you should first draft a specification of goods and/or services. This describes the goods or services for which you are using the mark. It needs to be sufficiently clear and precise and cover not only products of immediate interest, but also those which are intended to be of future interest.
To identify which category your goods or services belong to, you should take into account:
the function and purpose of the goods and or services
whether the goods consist of any raw materials
what activities are involved in the service, and
the class to which the subject matter of the advice corresponds when information services are being considered
The Trade Mark Act 1994 allows you to specify territorial and other limitations, such as waiving the right to the exclusive use of any part of the mark. In some cases, those limitations can increase the chances of success of the application.
It is not in the interest of a brand owner to make the specification so broad, so as to cover goods that the business does not, in reality, anticipate to market, since this would risk the mark becoming vulnerable to an attack post-registration. The application must be supported by a statement that the mark is in use in relation to the relevant goods and services, or that the applicant has a genuine intention to use the mark in such a way.
You can use the government's search UK trade mark classes service to search for and classify the goods and/or service classes you need to register your trade mark.
For more information, see the government's guidance.