I agree this will be seen as 'odd' or 'unusual' so the OP and their partner may be subject to additional scrutiny.

People flying into say LHR then out of LGW is not that unusual but into MAN then out of LON could raise some flags. Are there any airlines that fly from the USA into MAN but not LON?

There are lots of flights to europe from MAN as well so that will add to the flags.

Another is that you ex-UK into Europe flights will be on a separate PNR as you have not even booked them yet.