Well... following on the Gathering of the Clans, it may be that the stocks in many shops are a wee bit depleted. I'm guessing you are an American (you don't say where you live) and therefor you probably aren't aware that UK sizes don't exactly run true to US sizes-- most shops will want to measure you before squeezing you into a jacket. You Might think you are a 46 regular, but the tape measure might suggest a 48 regular would be a better fit.

Probably the best place to try for ex-hire would be Moss Bros., who should also have your size in stock. If they don't, they can usually supply your size from one of their other stores within in 24/48 hours, although there may be a charge for rush service. Moss Bros. has stores in Edinburgh, and throughout most of civilized Great Britain.