If you get a portable one, make sure you get the kind with two hoses. Otherwise you'll create a vacuum that will pull in hot air to the room you're trying to cool.

The second hose prevents that by using outside air instead.

Really thinking about investing in one of those ~£300 air conditioning units for home next year, as summers seem to keep getting worse, anyone tried those?

I'm not keen on the idea, air conditioning makes it feel even hotter when you go outside - and who wants to sit around inside on a hot day?