You can upgrade at the airport using additional miles although sounds like you dont have that as an option. As for using cash then the problem is a miles ticket does not have a money value so it is not as simple as paying up from an economy to upper ticket. You can do this with miles.
The only way I think it would work is for you to buy a ticket and use the miles to upgrade it but another problem here is that you would be buying a one way ticket (NY - UK) and therefore that would be a full fare Y ticket which will propbably set you back US$900 each - then you can use 10k miles to get from Y to PE or 30k to get from Y to UC
But there is another problem - as you have already booked I dont think you can cancel and have your miles refunded to use differently.

The best suggestion would be to call customer service and see if they would let you buy a revenue ticket from UK - USA roundtrip in economy and then apply your miles to upgrade to PE or UC. They may not allow this, it will cost you a fare bit of money and probably will not be available on the route you mentioned as it is peak summer time (NY - UK might be better as there are several daily fights).

A final suggestion in NY - ask the check-in agent if there is any chance they can do something as you just got married. You might just get lucky.

As for lounge access - only available for UC ticket holders and Gold card holders. Someone here will correct me if I told you wrong.