A VPN protects you by encrypting the connection between you and the internet. In the UK, it is very unlikely that your Internet Service Provider (ISP) is up to no good. However, if you use public WiFi in a bar, coffee shop or other untrusted location, you have no way of knowing what software may be running on it to steal your data and cause you problems later.
A VPN means that every bit of traffic from your computer to its ultimate location is secure.
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