Well there area couple problems with this. The first is, which El Salvador? The oneworld timetable only lists El Salvador, Chile which would not be in NA.

The other problem is that once you reenter the county from which you departed, you cannot leave again. For these purposes Canada and the US are considered the same country. Since you left the US and then reentered from LGW, you are not allowed exit the country again.

So if you were talking about the city San Salvador in the country of El Salvador, which I suspect you might be, then this too would not be allowed unless you can get there without entering the US which is not possible as best I can tell. Now you can't go there, but you could go from LHR to NAS, GCM, or MEX on BA, and a few other destinations like BGI and SLU I believe. Iberia flies from MAD or BCN to destinations in Costa Rica, Mexico, and Guatemala. I could not find a route map for Iberia so, without doing a lot of work, I don't actually know to which cities in these countries that they actually fly. Their web site listed these three NA countries in addition to the US.