Cycling Time Trials (CTT) is the authoritative body for time trialling in the UK. It has a highly active and forward-looking membership, and this is reflected in its informative and up-to-date website. A handful of events allow online entry; you can filter for these on the main CTT website, or go to the Online Event Entry portal to sign up and enter. (Note that payment is by credit or debit card, and there's a 70p surcharge as a result.) By the way, for thousands of photos of British time triallists going back through the ages, have a browse and search of ttlegends.org.
08 Sep 2001
19 April 2019 (Friday): Gt Yarmouth CC Good Friday 10-mile time trial