I suspect that it is true the Admirlal Fisher actually wanted to use the light battlecruisers as well-armed light cruisers

I read that at one point, Admiral Fisher had praised the Courageous, Glorious, and Furious as being good light cruisers. They would be able to maintain their speed better than the small ships, and would have a potent offensive capability at short range. They were not suited for long-range engagements at sea, but they could be extremely deadly at pre-dreadnought ranges, where they would being firing directly at a close target. The British excelled at this sort of firing in the late pre-dreadnought period, especially after Percy Scott's improvements. The Ger/CBL/1915 Mod.A is this sort of ship. It is fast enough, at 37 knots, to catch smaller cruisers, especially in a seaway, and has enough protection with a 4in belt to be able to engage them at close range. If 40 knots were achievable, this type would be even more capable.