Yes, the segregation is total, with different entrances/exits and facilities. Tickets for away matches are sold at the away team's ground. The segregation is enforced by both the police and employees of the club, called stewards. If the two team's particular rivalry is nasty, then they will leave an entire section of stand empty as a buffer between fans. A general rule is that away stands are behind the goals, while home fans have the sides.
Replays are delicate subject in football. There's been call for it for goal-line incidents, but nothing has really come of it as yet. Why? Bureacracy. You think it's bad for your sports, then you should try having governing bodies that span both the country, the continent and the world. The FA deals with national issues, but rule changes have to be approved by UEFA (the European body) and FIFA, the international one. Football is slow to change. I think the last proper rule change was about 15 years ago, when it was decided that a goalkeeper was not allowed to pick up the ball when another player from his team passed it back to him. It cuts down on time-wasting at the end of a match.