If you want to pay monthly, their RateSetter monthly charge is a mere £7.65 per month over two years with a £25 upfront charge. You then stick whatever goodybag you want on top.
The Galaxy A21s has a 6.5″ HD+ TFT Infinity-O screen (720 x 1600 at 270ppi), Samsung Exynos 850 octa-core 2GHz CPU, 48 megaxpixel main lens, wide-angled 8 megapixel lens, a 2 megapixel depth cam and a 2 megaxpixel macro lens. Up front is a 13 megapixel f2.2 selfie shooter.
The giffgaff network operates on the O2 masts and you can learn more about the A21s right here. Don’t forget that you’ll need to buy a goodybag of £10 or more to kick things off.