AA only seems to fly from DTW to LGA in terms of NYC airports. so by default if routing through NYC (so EWR or JFK) you will have to transfer between airports and that cost will be yours to bear. This is exactly the same as for revenue tickets.
In terms of a reward DTW-LGA/JFK-LHR in J I get the following pricing options:
65000 Avios + £ 519.40
58500 Avios + £ 654.40
49500 Avios + £ 809.40
43350 Avios + £ 934.40
38100 Avios + £ 999.40
33350 Avios + £ 1,064.40
The taxes/fees/charges are made up as follows:
Transportation Tax(Departure) - USA GBP 14.50
Passenger Civil Aviation Security Service Fee - USA GBP 4.30
Passenger Facility Charge GBP 3.40
Carrier imposed charge GBP 497.20
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Total taxes, fees,charges per person GBP 519.40