The infrastructure is there in the south. Good luck when millions of tourists descend on the north and need a train!
While I'm not familiar with the state of railway travel in Scotland, it's not indispensable that people travel by train. Planes, coach, car rental are all possibilities that surely can accomodate the needs of travelling tourists. I can imagine lots of Europeans could even take a ferry and come with their own vehicle from the continent.
I think we also have to be careful with the standards we expect from host countries. If absolutely everything has to be top of the line (hotels, transport infrastructure, stadiums, weather, football culture, political climate, country size, distance for European fans) then we would end up with a rotation of the U.S, France/Germany and Japan/China every 4 years. Yes Qatar, is a massive mistake from FIFA for so many reasons, but I think Brazil and Russia have shown that you can organize a top tournament even if you have deficiencies in some areas.
For the record, I think all the mentioned bids would would work very well. I think Portugal and Morocco deserve to host WC matches and bidding together with Spain would give them a good chance.
A UK bid would be fantastic. Think of the venues involved filled to the brim. But the FA needs to change their modus operandi this time. England could massively benefit from having the Welsh, Scots and eventually Irish doing some of the lobbying. They don't carry the same political connotations as an Englishman would.