But what's the deal with Ziploc bags? are they just really expensive? or what? I'm confused.
Tesco sells bags with zip closers, just not brandname Zip-Loc. The price is not shocking, IMO, but if your budget is student income based as opposed to grown-up job income based, you may think otherwise!
They only come in a small (sandwich), medium (really kinda big) and large (quite big)*. If you are hooked on particular sizes feature (freezer strength... I know there were other dealies don't remember them now!) then you should stock up on those. But for the basics, they are available at Tesco, which is one of the largest grocery store chains, and probably elsewhere.
*forgive the vague sizing, I can't be bothered to go look at what the containers say are the actual sizes. Besides, it'll be in metric and if you aren't here yet, you won't be able to visualize from that anyway!