An academic paper on 'Impacts of Power Grid Frequency Deviation on Time Error of Synchronous Electric Clock and Worldwide Power System Practices on Time Error Correction' published by
http://www.mdpi.com/1996-1073/10/9/1283/pdf
Based on data from sensors all over the world gathered in 2016, the authors found that Time Error Correction is practised in North America, Western Europe (including the UK), Japan and Australia but not most other countries monitored.
I have found nothing in National Grid documentation available online so perhaps it has been quietly dropped here? I don't know.