Why only on a Friday 8 cars Manchester to Edinburgh? Does the service not warrant it rest of the week? I often parallel run the 07:30 off the airport past Levenshulme and it's quiet at least as far as Picc.
Manchester-Scotland, particularly Edinburgh, has a weekend peak (Fri, Sun).
So TPE concentrates its 350s on these days to Edinburgh, and fills in with 185s taken from TP North routes.
There aren't enough 350s (10) to run more services without poaching stock from other services.
Virgin also try and concentrate seats on Edinburgh Fri/Sun from London/Birmingham.
Manchester Airport-Piccadilly is not representative of loadings you might see at Oxford Road/Wigan/Preston.