The website https://grid.iamkate.com/ gives instantaneous and historic energy mixes for the uk's electricity consumption. Over the last year the average demand has been 30.4GW and 43% of this energy mix has been supplied from fossil fuels. Currently almost all fossil fueled power stations release the combustion product CO2 into the atmosphere. CO2 is a greenhouse gas and causes global warming.
We need to eliminate fossil fuels from the energy mix to slow global warming and fulfill our international climate control obligations. It is not as simple as building an additional 43% x 30.4 GW of renewable energy resources. These are not constant producers of energy and in particular the wind does not blow all the time. Solar makes up only approximately one third of the supply, wind being most of the rest. Both of these are inconstant suppliers of energy. We need a source whose production is uncorrelated with these.
Tidal energy is in its infancy in the UK. There are (as of February 2021) only two operational tidal power stations operating within the UK: MeyGen, on the Pentland Firth in Scotland, is operational but not fully completed, while Bluemull Sound in the Shetland Isles is also operational, see https://www.saveonenergy.com/uk/renewable-energy/how-is-tidal-power-used-in-the-uk/
Storage of energy is difficult and expensive. It is important to have a mix of sources that are essentially decorrelated over time so that there is reasonable constancy in the combined output. Solar and wind are decorrelated, tidal energy would add a further decorrelated energy source which is more predictable over time than either solar or wind. The potential for tidal energy in the islands of the UK is huge.
Sea levels are currently rising at about 3.6mm per year, see https://www.climate.gov/news-features/understanding-climate/climate-change-global-sea-level, and accelerating. The causes are mostly global warming and the melting of stored water in glaciers and ice sheets at the poles and elsewhere.
An interactive map from climatecentral.org shows the current areas that will flood with increasing sea level. A google earth visualisation shows this in 3-D for certain, at risk, cities across the world including London.
The UK is a relatively small island with a large coastline. The furthest distance of any point from the coast is 84 miles and we have a total of 1130 tidal estuaries (see https://sac.jncc.gov.uk/habitat/H1130/map ) up which the sea flows twice a day. The effect of rising sea level will be felt most in the areas surrounding these estuaries. The only UK estuary to currently have a tidal barrier is the Thames.
There are many disadvantages to a tidal energy barrier if this solution is looked at in isolation. Estuaries are homes to much wildlife, the cost of tidal energy is much greater per Watt than wind or solar, the engineering difficulties of building a protection barrier that needs to span the whole estuary are considerable.
But when looked at as a necessity to protect cities from inevitable sea rise and increased extreme climatic events alone, an estuary spanning barrier becomes essential and, unlike the Thames barrier, why not include energy generation at the same time? The benefits of such energy generation are significant - tidal volumes are huge, largely predictable and decorrelated with the other main renewable resources. Once built, the maintenance costs are relatively small. In a country with relatively small land area but many large estuaries tidal barriers with incorporated energy generation is a good use of resources.