One of the main advantages of the shoe lump is cost.You can use bolt action barrel's which are considerbly cheaper than chopperlumps and as Twobobbwana say's,you can profile them before assembly saving time.It's a good production method.
As to the use of the Purdey action.Purdey's use it and don't seem to have any problem's.The 375 action will be plenty strong enough for your 318 Westley and at least at the end of it you'll have a rifle that you know will work and will be unique.