[Daumic (OP)]

With the search for oil and gas in the North Sea, significant coal deposits were discovered in this sea (1). 

The localization of these deposits in a sea-bed prohibits a traditional exploitation by mine. In situ combustion was considered with an aim of generating a combustible gas mixture easier to extract. But this technique was too polluting.

There is perhaps another resource to consider: the methane adsorbed in the pores of coal. This type of gas is called CBM for Coal Bed Methane (2). Several data could promise a significant and exploitable gas resource:
-   the amount of coal present under the North Sea seems significant; the amount of adsorbed methane should be in proportion,
-   part of these offshore coal deposits are sufficiently close to the coasts of England to be accessible by terrestrial drillings,
-   the technique of horizontal drilling, already used to recover shale gas, can be employed here to extract gas from coal,
-   coal is a porous rock, therefore the extraction of gas does not require hydraulic fracturing. 


(1)   https://deepresource.wordpress.com/2018/12/18/north-sea-ucg/ [nofollow]
(2)   https://en.wikipedia.org/wiki/Coalbed_methane# [nofollow]

[Bored chemist]

A classic example of "just because you can doesn't mean you should"

[paul cotter]

Not so quick, BC, you might need it to keep your Victorian property warm!

[alancalverd]

The Lurgi process for underground gasification of coal generated useful fuel in Germany during WWII and was trialled for some experimental UK power stations in the 1950s.

Several Northumberland coal mines ran out under the sea in the past. Most of them were considered irrecoverable post-Thatcher as they require continuous maintenance and will have been seriously damaged by flooding within weeks of shutdown, but the UK is now moving towards a wartime economy where state capital investment in strategic resources should be reviewed.

[Petrochemicals]

We can, basically you are describing fracking, but the question is, is it economic to do so. If you don't care the price point of your fuel Canada has the largest oil reserve on the planet, it's just the fact that it is so expensive to extract. My guess is that the cost of fracking plus the cost of oil rigs renders it too expensive.