I'm thinking of getting messy with some porridge oats and am not sure if I should cook them or not, will it make them thicker / cause them to "swell" or is there no difference?

Obviously I'll let them cool down before the mess commences....

ive always cooked them, alwaysbeen amazing, only trouble is cooking enough

Best cooked. Also, they are nicely warmed up by doing so as previous poster suggests.

Is quick oats a thing where you are?

These are oats that are ready in just 1 minute.

If they are, then literally, all you have to do is add hot water and let it cool.

All of my porridge sessions were done with just hot water poured over, cooled, then mixed.

If you use cold milk & large bowl & leave overnight in the fridge to allow oats to absorb the milk results ought to be fine.

I suspect 'porridge oats' are similar to what we call 'oatmeal' in the states... a whole grain rolled oat you mix with water and boil for a few minutes?

In that case, I'd definitely say cook them. Me, personally, I actually over cook them a bit...and add some extra water and a decent dose of sugar, though not so much for taste. The overcooking brings out some extra starch from the oats and the sugar/extra water makes a bit of a slimy / slippery goo ... with the warm, soft, mushy oats mixed in. Definitely a different feel from almost anything else messy, and a real treat to slip an slide around in!