Prounciations in British English?

I have just found out that St. James (or is it St. John?) is pronounced "Sinjin" in British English. I'm appalled and surprised I did not know this before as I graduated with a Brit Lit degree from an Ivy League university and had no idea it was pronounced this way. Can anyone tell me the history behind this and does this refer to peoples' names or does it include actual saints, place names, etc. If you would include any other examples of this type I'd appreciate it (such as why is "Ruthven" pronounced "Rivvin"). I just find this fascinating.