"what provision is the UK government currently making for the loss of it and the decommissioning costs"
Since the UK has never had a genuine Energy Policy it's what you'd expect - keep whistling a jolly tune and kick the can down the road. On Abandonment the liability sits with the current owners of the facility who are supposed to keep an up to date estimate of what it will cost (due to major advances in technology and actual experience these costs have actually been falling over the last 20 years). In addition when someone like Shell sells to a venture capital based start-up in Aberdeen they either give them some cash or retain a proportion of the liability
https://www.gov.uk/guidance/oil-and-...-and-pipelines
https://oilandgasuk.co.uk/product/de...nsight-report/
the later is good introduction to current issues