What limits an early 26-knot battleship's boilers?

Would boiler metallurgy or feed-water purity be the main obstacle to the proposed high-speed British battleship?

Metallurgy, tube seals, and welding are the main limits; feed-water purity can be handled with purifiers and distillation plants.

Feed water could be managed with purification and distillation equipment. The more serious constraints are metallurgy, tube seals, and welding, all of which must withstand the much greater machinery demand. Reaching the target would require either an earlier workable small-tube boiler or a very large conventional boiler plant. Britain possesses relevant theory and engineering capacity, but achieving the speed would be costly and technically aggressive.