Can you screenshot what you see when trying to access the site please? In terms of error message or whatever.
In 99% of cases I've seen so far, this is because you have parental controls on from your ISP - take a look at this website:
Report incorrectly blocked sites on UK ISP and mobile networks
www.blocked.org.uk
We're currently blocked (if parental filters are active) via BT, Virgin Media and Vodafone.
There are otherwise a few automated blocks in-place at the firewall level for certain types of requests, based upon browser profiling - we've had several large DDoS attacks over the years. Generally speaking, it's worth seeing if another browser (without VPN) allows you to access okay too.