Originally Posted by callum9999

Well obviously not those exact words, but I specifically said that they could fly from MQT to IST via LHR for 30,000 AA miles - which is what you corrected me on!

Yes they do. It would make more sense to use them than 10,000 AA miles yes. Surcharges would be £17.50 via BA or something like £62 via AA. So if they wanted to see London, Paris and Istanbul, the best route (to me) seems to be:

MQT-ORD-LHR-IST: 20,000 AA miles + taxes
IST-LON: 10,000 BA avios + £17.50 or 10,000 AA miles + £60 or about £100 cash
LON-PAR: About £40 on Eurostar
PAR-NYC/LON-ORD-MQT: 20,000 AA miles + taxes

I don't have an AA account so can't check the taxes, but as the BA surcharge on LHR-IST only appears to be about £10, I'd imagine it won't be much more than only booking that first stage to LHR.

EDIT: And now I double check, the connections on the PAR-MQT bit look pretty dreadful! They don't seem to be able to get you back to Chicago in time to catch a flight to Marquette on the same day - so you'd need to overnight in either London, New York or Chicago. It would probably be better to pay the extra taxes and leave from London instead - which can do (unless you'd like to stay in New York for a night).