I have previously purchased several computers from PC World UK, unfortunately on multiple occasions their staff aren't always the best when it comes to selling a "PC". By the looks of it, you bought a sub-£200 laptop with 32GB/64GB eMMC storage.
Am I right? If you have a spare £50-£65, you can buy a Samsung (1 or) 2TB external portable HDD (-https://www.amazon.co.uk/Samsung-Slimline-Portable-Hard-Drive/dp/B008PABFX8-). Or a slim design USB?
If you cannot, or plan to move around a lot, then I recommend utilising the cloud storage offered by Google. In particular, because they are flexible, offer the most value and provide good quality apps for Android and iOS. If you have a Google Account:
For documents and multimedia;
- Google Drive (inc. Docs) and Gmail offer 15GB shared online storage.
- Google Photos (App and Web) can be used without limited, if you upload photos in a compressed (reduced) format.
These settings are available on the Photos app or from their website (-https://photos.google.com/settings-).
For music, you guessed it, Google Play Music (Free) - signup requires CC / Address, but
you will not be charged unless you go Premium. You can easily upload up to 50,000(?) songs (files) to their cloud, for free, which allows playback from Play Music app or from web / desktop (Chrome).
You may be able to reduce Avast, by not installing components you don't need, but that's your decision, since you bought a license.
Hopefully these tips will help in some way, and forget about the space occupied by Avast.