I don't know about using 5.0gr of Unique behind a 265gr bullet in a Webley... as the heavier the bullet, the more impact it will have on pressure spikes.
For 45 ACP, Lyman's pistol & revolver handbook lists a starting load of 5.0gr of Unique behind a 200gr cast semi-wadcutter for a total pressure of 7,700 psi and 670 fps (and this is where you're best to stay for safety reasons). For the same caliber & bullet, a max load of 7.5gr of Unique is said to yield 16,600 psi and 980 fps (already in the "too-hot-for-comfort" danger zone). For safety reasons, you should never exceed 12,000 psi in any medium & large frame antique.
It is important to note that nowhere does the Lyman manual give any loads for a 265gr cast bullet (neither in 45ACP or 45AR)... so you may want to tread carefully with this kind of a heavy slug, as it is approx 30%+ heavier than what is listed (and may be dangerous in conjunction with the kind of loads you're referring to). A 265gr bullet should be probably propelled by 4.5 of Unique MAX, unless you have some solid & reliable data to prove otherwise (note: the fact that your gun hasn't blown up yet doesn't count! Think micro-fissures...).