You can get basic information about a UK limited company from Companies House free of charge. That will show you how long the company has been in existence, any previous names, whether they've filed their accounts by the due dates and the names of both past and current directors. Further, you can then click on a director's name to find out about any other posts he holds (or has held in the past). That can be particularly useful if, say, you find out that a director of a firm which claims to be able to get you compensation from timeshare companies is actually a director of a dozen different timeshare companies himself:
https://www.gov.uk/get-information-about-a-company
Payment of just £1 will also give you access to, for example,the company's accounts (as filed at Companies House). That can be useful if you're worried about a firm possibly going bust or, for example, by showing that a firm advertising as if it's a multi-million pound international business is actually just one guy trading from his spare bedroom.
Google is, as so often, also your friend when looking for information about companies. Simply enter the name of the company, together with the word 'review' into Google and see what comes up. That will often lead you to reviews on the following sites although, of course, you can also try them directly:
https://uk.trustpilot.com/
http://www.feefo.com/web/en/
https://uk.trustspot.io/home/index
https://www.reviews.co.uk/
When considering buying from an unknown website a simple 'Whois' search can often give you a few clues about it as well. For any web address ending in '.uk', go here:
http://www.nominet.uk/whois/
If you find that the registrant's address has been hidden,don't use the firm. (Only non-trading individual are permitted to hide their address. If a trader seeks to keep his address secret, you should be very suspicious!).
For other web addresses, try here:
https://whois.icann.org/en
(That's not always helpful but you can sometimes get some information from there).
A bit of detective work, such as using Google Street View to take a look at the trading address of the company can also be useful. If the company offers philatelic services it might not be too worrying if you find out that it's just a terraced house. However if the firm claims to have thousands of TVs, fridges and washing machines in stock, you might ask where they all are!