Hey from the UK!
Well I am not sure if this helps or not, try checking out the transat website (www.transat.com or .ca). For cheap flights from UK to North America. Once in the US or Canada try american airlines (they have an Olivia discount - you may have to drop the first A off the given code to get it to work online...). or check other carriers. I am from Canada, but right after Ixtapa I am heading to the UK to visit family, it would be great to meet in Mexico! Also, flying through Dallas Fort Worth, Texas on AA can get you to Ixtapa at 1:00 in the afternoon (more time in the sun!). It will probably be cheaper to book portions of your travel seperately. It ends up being cheaper but can get you in trouble if you miss connections so I would plan a LOT of time between flights especially where you need to clear international customs. Hope it helps!