I could be wrong, but I'm guessing that it could be one of those operations where the crew work 6 months stateside, 6 months Europe. Apart from the Miami Air markings, did you happen to notice whether the aircraft had an N- tail number or some European registration?
I do happen to know that there is an agreement between Miami Air and Excel Airways of the UK to do this kind of thing. So...it may have been a Miami Air B737-800 operating on behalf of Excel for the UK summer season. I doubt very much if it was a revenue flight all the way from Florida to Greece even for a 737NG!
