Getting us in meant adapting and creating laws, in a way that many previous treaties, e.g., military alliances and the like, didn’t, and crucially writing into our statute books that in the relevant areas, EEC (as it then was) law had direct applicability.
In 1970-72, there was prolonged debate about whether or not we should join (though this had been an issue since the early 60s, de Gaulle’s obstruction had prevented an agreement). The Heath Government came into office at the 1970 election committed to another application. The Treaty of Accession was signed after a positive vote in Parliament to approve the terms, and the Act of 1972 gave legal effect to it. It can’t be undone without Parliament doing something about the 1972 Act and all the consequent EU regulations that currently have effect in the UK.
The question turns on whether triggering Article 50 has a legal and legislative effect, among them the implicit removal of individual rights (such as freedom of movement) acquired as a result of the ongoing effects of the Act of 1972. The court’s view is that because of its irreversibility, it must (see below).
Possibly, there are all sorts of polls indicating enough “buyers’ remorse” to suggest a second referendum might come to a different result, but without much more precise terms we don’t know how people will react, and we wouldn’t know what the terms are until after Art. 50 is triggered, by which time it is too late to row back - see below.
Moreover, the political opportunities and mechanisms for such a change of heart to become politically effective are unclear, absent a series of parliamentary by-elections to give us a clue. There is one pending in an apparently safe (but strongly pro-Remain) suburb of London, where the local MP is a Brexiteer but has triggered the by-election because of his promise to do so if the government decided to expand Heathrow airport, which they have. So reading the entrails of that result will be an interesting exercise. Otherwise, we may have to wait for unforeseen accidents and heart attacks…
On the specifics of the importance of one issue over another (whether immigration or farm subsidies, or research funding, or… the list is rather long)… the government is still going through a balancing act, but the PM appears to have made immigration the top issue at her party conference (and of course EU membership has absolutely nothing to do with immigration from outside the EU, which is entirely within the UK’s control already - but if we were to change our minds at the wrong time, who knows what the other member states might expect of us before allowing us back - see below)
Then again, what’s said to satisfy the troops at party conference doesn’t always come to life in policy-making reality.
The text is pretty clear. It’s irreversible in itself, and has effect on completion of an exit agreement (which doesn’t necessarily have to be a full outline of the future relationship), or two years, whichever is the shorter. This can only be extended if all member states agree to do so.
If the withdrawing state wishes to reverse the decision having once triggered Art. 50, they have to re-apply under Art. 49 - and another way of looking at what Lord Kerr said (and he drafted those bits of the Lisbon Treaty) is that it’s by no means certain what conditions the other member states would place on that, or whether any consequent Treaty (which is what a new application amounts to) would get approval by each member state’s own processes (some member states require all new EU treaties to be approved by referendum).