[Frozen-Stick]

I wonder if it shouldn't be possible for companies in UK to store NG in old oil or natural gas fields, which were tight through millions of years and now only do have some drilled holes in it?

Perhaps you only have to change some valves. All the infrastructure still exists (pipelines etc.). This shouldn't be a case of huge amounts of money, but it should be a case of responsible energy policy to give some time for preparing for declining energy ressources.
What about a giant government program to subsidy all efforts of insulating homes with an already existing and sophisiticated technology?

[lawnchair]

Possible, yes. Though there may be more accessible (on-land) depleted fields for storage.

Realistically? Fill it with what? Do you know of some source of cheap, plentiful natural gas that the rest of us were unaware of?