Even though the 'Welcome to Wigan Borough (twinned with Angers!)' sign is about 200 metres along Upholland Road, before the row of semis on the left (on the right if you're coming from Orrell), that row of semis are in St.Helens. So they mustn't have moved the signs yet.
If you ask me, it's logical to use the M6 as a boundary between Wigan Borough and St.Helens, and the M58 as a boundary between St.Helens and West Lancashire.
Although, on second thoughts, no! I think these 'boroughs' are too big. The outlying areas all suffer to the more centrally situated areas, especially in Wigan Borough.