Hi!
An iPad is the best Apple (and tablet judging by all the reviews) investment you can get for STEM, in my opinion. I have pretty much every device Apple offers, and I can say if you're looking for an affordable investment, an iPad is the way to go.
I know, Apple is expensive, but you have to consider these as investments. I'll assume you want to go for an 11-inch M2 iPad Air. This is an incredibly powerful iPad, and definitely one I recommend. It's typically £599, but with UNiDAYS (which you can get access to if you use a college ID and manually take photos of the college ID or use someone else's), you can pay £549 AND get an £80 gift card during the student summer promotion period which occurs every year (enough to cover the cost of the USB-C Apple Pencil!). You can't use the gift card on the iPad purchase, unfortunately.
However, the positive is you basically get a free pencil! I'll also assume you intend on using this very iPad for your sixth form duration, and your degree (~6 years), and with maintenance you can definitely make it last that long.
Upgrading storage will cost more, but you can also pay for extra iCloud to limit your storage use.
It's a big chunk of money to throw onto a device, but an iPad is versatile and helpful. If there's one device you buy, it should be an iPad. You can also pay for a bluetooth keyboard (since the Apple ones are disgustingly expensive) and that can double as a laptop in a way, and then you can type up your practicals and essays etc.
BackMarket is very reliable, I would advise buying excellent quality since you want to prolong the lifespan of the device so it'll last you through university, too. It's a big cost for a long-term benefit.
An iPad is a multi-purpose device, and it has saved me so much money in printing and saved my back from carrying so many folders and books because I can digitise it. I was able to do past papers in the car, in bed etc. when I felt lazy or couldn't be at my desk.
It's convenient, reliable, and durable.
Happy to answer more questions!