[dmurray]

I suppose it would raise a few eyebrows to take off from City and do its refueling stop 20 miles west in Heathrow.

Of course Heathrow is also far busier than Shannon and far more likely to have delays.

[gsnedders]

Heathrow also doesn't have US preclearance which it relies on to justify the longer flight time due to the stop. (And it would be an utter waste of a slot at Heathrow: why would you use up one of your slots at Heathrow for a refuelling stop when you could instead fly to an airport which isn't slot limited?)