The Itoya 18x24 portfolio is the way I store my unframed prints, keeps them nice and flat and has acid free paper in each sleeve.. This is a quick Amazon UK search, but it looks a little different to the one I have :
https://www.amazon.co.uk/Itoya-Origi.../dp/B00009R8XD
Sorry that's out of stock atm on the UK site here's the US link:
https://www.amazon.com/ITOYA-Present.../dp/B00009R8XD, I got mine from the USA, a kind member here sourced it for me but i can't remember who (thanks whoever that was!!!
)
EDIT- nipperkipper was the awesome member who sorted me with the itoya portfolio, thanks again my friend!
For larger prints I'd suggest an A1 or even A0 portfolio, these can be readily found in the UK as it's a UK standard size, it's only the American prints that are a weird size because of the Imperial/Metric difference in measurements. Oh and Japanese prints are also different sizes but closer to the UK standard. The 18x24 US prints are a real pain because they are slightly bigger than A2 but massively smaller than A1. You have to buy custom made frames for them in the UK and that gets expensive quickly.
I'd advise against storing prints in tubes for long periods of time, you'll never get them flat and the cardboard tubes are not acid free usually so could potentially damage/fade the colours. PLus having a flat portfolio is sooo much easier to store than lots of tubes.