Hi mvfoe23,
We have been on SCP last september and paid about £240 to upgrade for 2 adults. I havent flown from Manchester on SCP or with a family of four but in my opinion I think that is quite expensive because that works out about £350 each. We are going on the 27th September and have upgraded this time and paid a silly price of about £60 each but we booked last October and I think FC mucked up their prices that day!!
As you are going soon, your price might not change and it might be a risk as Premier seats may go but I think the £350 each is a bit much