If the result is to stay in, nothing needs to happen.
If it’s out, a lot will have to happen. Here’s a non-exhaustive list:
The U.K. will have to abrogate several Treaties –Rome, Maastricht, Lisbon, to name but three. This will require at least one, if not several Acts of Parliament, which will have to go through the due constitutional process before receiving the Royal Assent. I’d guess that the Lords will send it back to the Commons as often as they can.
In parallel, we will have to embark on the technical process of withdrawal. I have no idea what this entails in practice, and I suspect that nobody else does either. There may be some procedures written down somewhere, but they’ve obviously never been used in anger. So it’ll take some work to turn theory into practice. We’ll probably need a new department of the Civil Service to unravel it – and the Foreign Office will certainly need to be drawn in too; after all, we will also be renegotiating every single EU treaty with every other country we’ve signed up to, one at a time.
Also in parallel, we will have to assess every single law, regulation and protocol that has emanated from the EU (or its precursors) and decide which of them to retain and which to repeal. Given that these are enshrined in U.K. law, many Acts of Parliament will, again, be required, and will have to go through due constitutional process.
Then – and this can only happen once all the above has been performed, vetted and reviewed (not to mention publicly debated) – then, a single issue General Election will have to take place. It’s impossible for this to happen in less than two years, and my guess is that it’ll take nearer eight. By which time all the current generation of politicians will have faded away.
I have more!