Let's not forget what is being suggested here, clean renewable energy which is promised to be much more predictable than wind and solar. Reading the previous comments about predictable vs dispatch-ability are spot on, but maybe forgetting one thing. The issue with current renewables is not that we don't have enough power, the big power stations produce more than enough most of the time but it is key in the distribution infrastructure that the National Grid control the amount of power entering and leaving the network (supply and demand) and the predictability of large amounts of tide energy coming onto and off the grid compares favourably with balancing renewables production with nuclear and other sources putting power onto the grid.
The cost may be high for consumers initially, but then again new technologies in the power industry always are. There is an opportunity with tidal power to generate a huge amount of electricity in the UK, with no CO2 or other nasty things like radioactive waste. I honestly think that "green" technologies will always be significantly more expensive to run and maintain than traditional ones like gas/coal/oil and even nuclear, but the fact is that the additional cost is worth paying to look after the environment. It may not offset the cost to consumers, but in terms of government backing for this I could see the lagoons having a positive impact on tourism in the areas they are built too.