Do this on Companies House.
Simply;
Pick a Company Name,
Fill in the details,
Select a SIC code.
(a SIC code describes the type of economic activity in which the company is engaged).
Companies across the entire UK - England, Scotland, Wales, and Northern Ireland, use the same UK Standard Industrial Classification (UKSIC) system
Here’s a link for you to find what SIC to use;
You can pay an accountant to do this, or,
Do it yourself online for approx £100* (edit thanks Mirka Fris)