What makes Companies House reject a company name?

The name must end in Limited or Ltd, and it cannot be the same as an existing name once punctuation and common words are ignored. Words that suggest a link to government, such as Accredited, need permission from the relevant body first. Offensive names are refused, and a name found too like an older one after a complaint may have to change.

Source: Set up a private limited company: choose a company name, GOV.UK