No clue why England got a 5.5, but I guess its because the semi-finals and the final are in London?
Yes, they have home advantage for these games, but they first have to get there

In fact they have home advantage in all games apart from a possible quarterfinal, where they would have to travel St. Petersburg/Munich/Rome/Baku.
The odds are low, and yes, I do think they are too low as well, because there is a lot of betting people in England and they form the majority in Betfair exchange, where the market is made.
You can witness this in every major football tournament, where England is participating. The english punters will just bet on England no matter what. So bookmakers will offer very low odds for England all the time, since they know, that their english customers would more or less play any odds for England and that will give the bookie some nice value.
Edit: In fact England doesn't have home advantage in R16 too.