The UK implemented electronic exit checks on 8 April 2015. On March 28 of last year the Independent Chief Inspector of Borders and Immigration published the report An inspection of exit checks, describing an inspection that was carried out between August 2017 and December 2017.
The report's "summary of conclusions" begins with an overview of the history of UK exit checks from 1994, when they were first relaxed. It notes that before 2015 there was already a system called "Semaphore" for the collection of carrier data on passengers. The program developed a new system called the initial status analysis system, or ISA, which "matches inbound and outbound travel data received via Semaphore with data recorded on other Home Office immigration-related systems."
Confusingly, the report notes that the exit check program ended in May 2016, but that the ISA continues to operate. The report concludes that the program should have lasted longer, to allow the development of additional uses for the data collected by ISA.
On the subject of cooperation with Ireland, which is necessary to record departures of travelers who leave the CTA through Ireland, the report says at paragraph 3.15
When exit checks were announced, the Home Office stated that “alternative arrangements
[were] being put in place to cover journeys made within the CTA”. In March 2016, the Irish Government passed legislation that created the legal basis for the Irish authorities to provide API on CTA passengers to the UK Government. However, at the time of the inspection, this was still not happening routinely.