Why Britain could not simply send battleships to the US Pacific Fleet

Would British battleships have reinforced the United States Pacific Fleet after Pearl Harbor?

Probably not; Britain first needed those ships for its own security and the Eastern Fleet before it could reinforce an ally.

The hypothetical asks whether Britain could send Revenge-class battleships or Warspite to replace American losses. The Royal Navy first had to protect its own positions and assemble the Eastern Fleet. An alliance does not erase each member's obligation to secure its home and imperial commitments before releasing forces elsewhere. Allied strength on paper cannot be treated as a single freely transferable pool.