I'm not sure Community is the right place for this, and I'm no BA expert, but I do think I have an answer for you.

A round-trip award on the Concorde is 125,000 BA miles. Alternatively, for 95,000 BA miles, you can fly one-way on the Concorde and return in economy on a traditional jet.

Marriott points can be exchanged for BA miles, but this is where an exact calculation of your Marriott points is required. To get 125k BA miles, you would need 2 566 awards (125k Marriott = 50k BA * 2 = 100k) and 1 565 award (75k Marriott = 25k BA) for a total of 325k Marriott Points. Similarly, the 95k BA miles would require 2 566's (250k Marriott total = 100k BA total) for a total of 250k Marriott points.

In reading the BA board, someone implied if you book your award directly through the BA executive club desk, you may be able to redeem just a one-way Concorde award JFK-LHR for 80k BA miles, which would require a 566, a 565, and a 789, where 215k Marriott points would get you 80k BA miles. Then you could get a NW award for the return and any domestic connections.

All of this is convoluted, and it would certainly make it easier if there were a way to transfer miles out of NW to a program that could get you a BA award, but unfortunately that doesn't appear to be the case.

Note that this answer only applies to a solution strictly within the terms and conditions of the programs. I'm sure others may offer alternatives that may or may not be technically permitted but would still work.