If you are not in the UK, then the name of a company in the UK is totally irrelevant. My own company (in the UK) has a name identical to the name of a company in the USA, there's no problem. (Should they try to make trouble, I'll tell them to **** off).
If you are in the UK, go to the companyhouse website (for non-UK people: Companyhouse is responsible for all the registrations of companies in the UK). You can search for similar company names, and they have published the rules what names they will accept. General rules, plus rules for similarities with existing company names. You can always try to register the company if you don't understand the rules, and either you succeed and you're fine, or you fail.
But all this goes by name only: It doesn't help if another company is in a different industry. On the other hand, if you try to register "Voldemort Videogames", it's unlikely that there is a company with that name in another industry. And it doesn't clash with "Voldemort Car Cleaning".
However, with "Voldemort" in the company name, you might have a trademark fight on your hands. And anyway, this is just about the company name, not about names for products. That's where trademarks come in.