Ok, all you need to do with tpl_shopping_cart_default.php is upload it to..
includes/templates/your_template/templates/

I'm hoping you already have a custom template set up. If so, the text in red should equal the name of your custom template.

If a particular size, colour, model etc goes out of stock, then this mod will disallow it to be ordered (providing you have it configured this way in admin). The customer finds this out but only after adding to their cart. The ability to add a particular type of product to the cart which is out of stock *won't* automatically be removed from the product page.

I'm not too sure about the currency problem. I'm also from UK but haven't had this problem myself. It may be that GB isn't set as your default currency? Check in Admin>Localization>Currencies to see. Other than that, I'm not sure what else to suggest about that issue.

Yes, all of my site is Zen Cart. I've added quite a few mods to it so it might look a bit different to a standard version. I've not changed the template much though. I guess I should do that sometime.