This has been achieved not by buying energy from elsewhere (certainly not Russia), but from a reduction in the use of coal-fired power stations to effectively zero. They've been replaced by combined cycle gas turbine (which has far less emissions), biomass (the largest coal-fired power station now burns woodpellets), and a massive increase in offshore wind turbines.

Here's the obligatory link to the UK's live electricity sources: https://gridwatch.co.uk/