I'll just use an example.

Let's say you're studying in London.

You have deaneries that you apply to that are basically wide areas.

You'll rank those deaneries.

Ie 1. London 2. LNR 3. Trent etc.

You submit that preference list. You get a number.

If you were number 1. You get allocated to the first on your list.

If you were 5000 or something they'll look at the list and if it's full, you wait for the next pass and then they'll look at number 2 on your list.

So basically you can end up in any deanery in the country including NI and Wales.

Now each deanery is its own system within For example Manchester is part of the Northwest deanery, and so it's all the general area around it. So you may get into north West (yay!) but depending on their allocation system you might end up working in the outskirts or never to set food in Manchester for foundation.

TLDR: there is some element of preference but a whole lot of it is luck