The Royal Navy didn't have enough warships, even with 2nd and 3rd tier ships being used, to perform all the patrols required. What they did have was a number of liners that were available, and enough light guns and crews to arm them.For Britain, at least, the allure for a full scale implementation, didn't last long.
Being rather thirsty engines, especially at speed.
But it was also a solution looking at a problem that could have been resolved with enough light cruisers.
As to where you'd get said light cruisers, I tend to think this where keeping mothballed second and third classes come in.
Not sure what you mean in reference to 'thirsty engines' - nearly all ships for the time were coal burners. England had coal, but had to import oil from Persian gulf. England had a number of older warships, and much to their chagrin, the U-boats found the slower capital ships to be great targets. They were too old and slow to be used with the more modern fleets, and when relegated to secondary duties they ended up getting sunk with a large loss of life (think Brittania). Or when one U-boat sank 3 cruisers sailing together in less than 2hs (HMS Aboukir, HMS Hogue, and HMS Cressy). Stupidity knows no bounds.