The Battle of Britain Five Months That Changed History, May—October 1940 by James Holland (z-lib.org).epub

However, from the British perspective, it was clear that large oceangoing
convoys could no longer travel through the Channel. A 30 per cent
loss rate was too high, especially when the convoy had not yet even reached
the hunting grounds of the U-boats. The answer to Churchill’s question was
that, from now on, ocean-going convoys to and from the Port of London
would have to go the longer, more arduous, route up the east coast and over
the top of Scotland.
That was a sensible but time-consuming solution for ocean-going
shipping, but there was no way round the problem of getting coastal cargoes
to and from London and to the south of England. Britain depended on coal:
it was the lifeblood of the power stations which provided the electricity so
that aircraft and almost every other war requirement could be made. The
south coast alone needed as an absolute necessity some 40,000 tons of coal
a week. Without coal, Britain would collapse. Yet the inland transport
network – the under-developed road system and the railway network –
simply could not cope with the demands that would be needed were the
coastal convoys to stop. Despite the considerable threat from S-boats and
U-boats, from the Luftwaffe and from mines, the colliers and trampers
would have to keep going.
These crews would be performing a vital and heroic duty for Britain in
the weeks to come.