Below are links to various ISP status pages.
123-Reg – https://www.123-reg.co.uk/support/system-status/
BT – https://www.bt.com/consumerFaultTracking/public/faults/tracking.do?pageId=31
Demon Internet – http://status.demon.net/
FastHosts – https://help.fasthosts.co.uk/app/systemstatus/
Heart Internet – http://www.webhostingstatus.com/
TalkTalk – Unfortunately, TalkTalk have remove their original Service Status page and replaced it with a Business Network Status page that does not always work – https://www.talktalkbusiness.co.uk/network-status/
Virgin Media – https://national.virginmedia.com/service-announcements/status
If there are any errors or missing entries then please leave a comment below with the details.