So Incase anyone is interested in the (very condensed) story.
-At MCO notice flight to PHL is late so will miss onward flight.
-AA agent allowed me to borrow her phone instead of waiting in line. I found a MCO-LGW LHR-BCN-PMI flight option I liked but agent on phone said she wouldn't be able to book me on any flight options that didn't contain an AA flight so I should speak to a desk agent.
- passed security and helpful agent booked me on these flights
- got to gatwick and found sooner flights which avoided a trip to Heathrow and was a 3hr layover rather than 10hour.
-gatwick agents helpfully put me on the flights I suggested
-ended up arriving 2hours later than originally planned.
Overall VERY impressed with all the agents I encountered. Particularly AA at MCO and BA at Gatwick.
Moral, as always suggested here... Head to the desk with a suitable solution rather than just a 'im going to miss my flight'