plus you will need a map (which will go out of date if you buy it too soon)
I wouldn't worry about the expense of a map which goes out of date before you start doing actual navigation. It's good practice to prepare dozens of plogs for various routes, weather circumstances etc. before you actually do a navigation exercise.
Compared to faffing about in the air because you're struggling with a concept of something (at 150-200 UKP per hour), maps are dirt cheap.
And I echo the comment about the cardboard flight computer. It's sufficiently sturdy to get you through the PPL and that's probably all you need it for. After that you can always get an electronic one, rely on spreadsheets, use navigation software or "upgrade" to a proper metal one. My cardboard computer is still going strong after six years.
Furthermore, a ruler marked in nm, and some kind of protractor (to measure angles) will be indispensible once you start preparing your plogs.
Plus an appropriate pen or pencil to write on your map: The default CAA ones are laminated so they require pens that will stick to that (and alcohol wipes or something like that to get the markings off). I personally prefer paper maps (Jeppesen VFR+GPS) so that you can use a pencil, and they're easier to fold. But you've got to consider that at your actual nav exam you'll be issued a map by the flight school so it's a good idea to become familiar with that.