Why do so many Bitcoin websites have such bad reviews I'm very interested in starting out trading with a cryptocurrency but I've noticed every single website seems to have a low Trustpilot rating, full of one stars claiming the website is fraudulent etc. Is this just a common, untrue thing amongst Bitcoin trading sites?
There are indeed a lot of fraudulent online exchanges and wallet providers. The same is true for displeased customers.  

I suggest you take a tour at https://bitcointalk.org and get to known which providers are trustworthy.
For trading cryptocurrencies, I'll suggest you start with Coinbase/GDAX or blockchain.info. Then based on what coins you are interested in, find another exchange that support them.  

This page is also very helpful, it list the biggest online exchanges. 
https://coinmarketcap.com/exchanges/volume/24-hour/