Use of intermittent sources means that an equal, reliable and instantly available capacity must always be on hand. In Scotland’s case this, to a large extent, is coal-fired generation (Longannet).
To allow what is widely regarded as the politically more acceptable renewals generation, this means that Longannet, which cannot be quickly switched off and on, continues to burn coal from which no contribution to our electricity needs is made – heat (and carbon dioxide!) literally going up the chimney to no purpose.
This, I suspect, is the major reason for the planned early closure of the facility rather than a relatively small transmissions cost.
(Dr) A McCormick