I think 40 H&K will be over the wing with a very poor view. Have a look at:

http://www.seatexpert.com/_VS_Airbus_340_600_V1.html

which suggests you need to be no further forward than row 42 to avoid the wing.

One thing I would advise is to sit on the right (starboard) side of the aircraft on the MIA-LHR leg. If the flight lands in a Westerly direction (50:50 chance depending on the wind direction on the day), the flight path takes you in directly over the city of London. In that case, the views from the right will be of all the major London sights and, particularly for an infrequent or first time visitor, are well worth seeing. If landing Easterly, you're over countryside and it doesn't really matter either way.