Northwest- LHR to MSP
 

Heading to MSP with NW in two weeks time with wife. Last time I flew with them was internally within the US and was able to pay at the gate to upgrade to C class from DTW to LAX (cost I believe around $100). Is it possible to to do this on transatlantic flights or indeed onboard the aircraft with NW, although I appreciate at nowhere near that price! Know it can be done on VS BA and wondering if anyone had any answers?!Thanks in advancetlg