Not clear what program mocean is trying to accumulate miles in. If it's AA, then on a LONEx, the trick of routing MIA-MEX-LHR so as to be able to accumulate AA miles on the transatlantic flight on BA is not a good one, since the MEX-LHR flight on BA in L only earns 0.25 base miles per flown mile. Just flying MIA-LHR on AA would be more lucrative.

If the program is not AA, then the MIA-MEX-LHR trip is unnecessary if the point is to earn miles on the transatlantic flight. Of course, if the point is to go to MEX rather than to maximize mileage earning, then go right ahead. Or if this is actually an A/DONEx being credited to AA, then it's also a good thing to do.