For many small households, 100 Mbit/s fibre is already a strong baseline for browsing, video calls, streaming and ordinary work-from-home use. If several people stream, game, upload files or use cloud backup at the same time, 250 or 500 Mbit/s gives more headroom.
Gigabit fibre, usually advertised as 1000/1000 Mbit/s, is most useful when many heavy users share the same line, when you frequently move large files, or when upload speed matters as much as download speed. It will not automatically fix weak Wi-Fi in a back bedroom, an old laptop network card or a poorly placed router.
Mobile broadband can be enough for a single person, a temporary home or a property without fibre. If you use it as your main connection, check both the speed tier and the monthly allowance. Some mobile plans are unlimited, while others state 5 GB, 15 GB, 20 GB, 40 GB, 50 GB, 70 GB, 100 GB or 200 GB.