More Admiral-class hulls would consume repair-yard capacity

Could British shipyards build six or eight Admiral-class battlecruisers simultaneously instead of four?

Potentially, but only by using yards that the Grand Fleet needed for maintenance and repair.

Britain possessed slips large enough for more capital ships, but nominal building capacity was not free capacity. Diverting repair and maintenance yards into new construction would weaken support for the active Grand Fleet, turning a production gain into an operational tradeoff.