Some further research indicates that your Webley (or all of them, actually) have "standard" rifling. It may be on the shallow side, but it isn't Metford rifling.
Metford rifling was, essentially an experiment, based on the belief that at the "high" velocity of the .303 round standard rifling would cut and tear bullet jackets. It is widely known, but wasn't widely used, and after a few years of the real world showing that regular rifling did not tear up .303 bullets, the British dropped Metford rifling.
As far as I can find, Metford rifling was never used in any of their handguns, only in some of their rifles, and was discontinued when it proved unnecessary.
Some sporting arms of the era also used Metford rifling (it does work) but the use of Metford rifling in sporting arms generally went away not long after the military dropped it.
Your Webley has standard rifling (though it may be worn) and from the description a heavy build up of lead in the bore.
Getting the lead out can take more than a bit of scrubbing. There are chemical compounds (including mercury) that will save you a lot of elbow work, but they are nasty, and very toxic so care must be taken, proper PPE used and disposal in accordance with your local laws.
Good Luck!
