The plan is to build six of these lagoons. They expect to gets these built and operational *before* the new nuclear power station is completed. When the six are built they will generate a similar amount of power to that nuclear power station at a similar price per MWh. The lagoons have an expected life of 120 years, whereas the nuclear power station has a significantly shorter one, plus there are no major decommissioning costs.
By building both nuclear and tidal (and other forms of generator), we spread our risks.
Not to mention the nuclear plant is a joint Chinese and French operation with the profits flowing out of the country. The tidal power station is a British one, meaning that the profits stay in the UK economy.