If it helps, it is included in CAP694 found at
The UK Civil Aviation Authority
CAP 694: The UK Flight Planning Guide | Publications | CAA
In the introduction it states
The Civil Aviation Authority does not supply copies of the CA48 Form, as it
is becoming more common for FPLs to be submitted direct by electronic means and by fax.
An electronic copy of the FPL Form is available from
www.ais.org.uk/aes/en/CA48.PDF.
So I guess they're just trying to move away from the actual paper form all together to stop a torrent of faxes.