Hello,
We are fortunate in our town that our ironmongers do still stock longer handles...but not the split wood YD that you mention...or the shaped T that I prefer. They have the barrel shaped T handle and the fixing of this T can sometimes leave a bit to be desired.
http://www.richardcarterltd.co.uk is the company that I associate with tool handle manufacture in UK. Suspect they may be a bulk supplier.? but I did once have a railway shunting pole handle from them as a single item but it was collected by a dealer who bought and purveyed other railway related tools from them. Just don't ask why I have had a railway shunting pole stood here in the hallway.
Out of interest a few years ago I was buying quite a few tools from Ramco who are clearers of ex ministry tools etc etc....they would have cleared and resold all the things you would be familiar with to kit out those in quarters. I did also have some T spade handles from them once too.!
Sorry not to come up with a definitive supplier of you specific quest though.
Clive.