The contract period is for one year, which amounts to a total payment of £162. Keep in mind, the SIM offers 4G connectivity, not 5G, though O2 launched its 5G services in October. You can pick up the same 100GB SIM directly from O2, where there's also an option to get the same amount of data on 5G, but both options cost you £25 monthly instead.
If you are looking for a 5G SIM plan though, there's also this equally incredible offer from Vodafone (also via Mobiles.co.uk) with unlimited data for £16.86 per month. Speeds are restricted to 10Mbps though.