Sorry for the long list of quotes, but I've been away most of the day and catching up. You've gotten some great advice!
I'm a very frequent traveler to the UK--both London, Edinburgh, and Northern England (where DD has attended Uni for the past few years). I think I've been 6 or 7 times in the last 12 months

I fly from SFO and do anything possible to avoid a layover in the US. My son is special needs so the issues are similar as to a baby--basically avoid as much as possible the potential for disaster (i.e. long days, delays, etc.). We've learned to avoid layovers with him whenever possible. I recently decided to do a one stop in Chicago as I was by myself to try out a better business seat and I *deeply* regretted it. I was delayed and with a huge snowstorm the day before, the airport was a mess. With a baby, it would have been much worse.
Since you've now decided to do premium economy I would highly recommend Virgin--which is a transfer parter for both MRs and URs. They have the best PE product out there, and they also have a direct from MCO to LGW. My daughter has done this flight a couple times as we do Run Disney races on her breaks and she'll fly back to school from MCO. She's also flown direct from MCO to Manchester, which is also an easy connection to the train (and to the line that goes to Edinburgh).
My rec would be to fly direct either to LGW or MAN. If you go to LGW, you can do the Gatwick Express into central london to Victoria--it's super easy and I find even more convenient than the Heathrow Express, which has long walks depending on your terminal. We've actually stayed at the hotel right at Victoria station, which is super convenient (ASIDE: we've also stayed at the Paddington Hilton which is super convenient to the Heathrow Express and the Raddison Blu at Manchester airport, which has great family rooms). From Victoria on the Gatwick Express you can then use the tube to connect to King's Cross where you take the train to Edinburgh. It's only a little over 4 hours from London King's Cross to Waverly on the quick train.
MCO to MAN is another option if you want to go straight to Edinburgh. I missed your original post so I'm not sure whether your main destination was London or Edinburgh. There is a train from Manchester Airport that goes directly to Edinburgh--if you time that one right it only takes about 3.5 hours. There are tons of others with one stop that take a little longer.
We've done all these options as Durham University in Northern England is about 1.5 hours before Edinburgh on the same line
I love taking the train, but FYI it is not cheap and as
@GZanta77 mentioned it can be much easier and cheaper to just fly on BA if you are going to Edinburgh on the way in or out. Be forewarned though if you add on a SEPARATE flight to EDI to a RT from MCO to LHR, you can't check your bags all the way through. That happened to me recently, and it was a PITA. I had a RT from SFO to LON and added a one way to Newcastle at the beginning of my trip. I had to collect my bags and recheck them. Total hassle, but my flight was really cheap with Avios.
To avoid this, I'd book one way tickets and add EDI on to one of them, taking the train back or from London to Edinburgh one way (i.e. MCO to LHR to EDI on arrival; train from EDI to London; spend time in London and then fly home direct LHR/LGW to MCO. You can actually do BA one way (the part with EDI attached) and Virgin the other to try out the better PE product.
BA doesn't have nearly as good of a PE seat though so my first choice would still be Virgin. We often have this issue come up when my daughter is going back and forth to school. She can either fly to Newcastle via LHR on BA or fly Virgin to LGW or MAN and take the train. If she's going PE, she always chooses Virgin and the train even though it's more of a hassle with the trains/tube/bags. If she's in coach, she just does BA.
As far as finding flights, you will want to be as far out as possible, but in my experience Virgin seems to have more consistent availability in PE than BA. You can find tons of economy in BA, but PE and Business can be difficult unless you are doing it exactly when they release.