Matt, generally accepted by who? W231 is not a very fast powder.
At that charge weight what type of pressure signs are you seeing? I'm sorry but I can't see any 38 S&W brass showing pressure signs at that low a charge and pressure/velocity.
I suggest spending some time on google, its where I found much of my load info (some nice posts on this forum). but some sources I can recommend are the Speer loading manual, Hodgdonreloading.com, and Lee's reloading manual.
as for you not being able to see brass showing signs of high pressure from this cartridge/gun combination... well I cant help you. hand load for a Webley and you will be able to tell by looking at the brass when things are getting a bit too hot.
-matt