Not all tidal power is in the sea. A Severn Barrage would be in an estuary and could generate up to 7% of the UK's electricity demand, depending on what location was chosen for the barrage.
We can't even build a railway between our #1 and #2 cities let alone take on what would be one of the world's greatest feats of engineering.
Quite. There aren’t that many estuaries and the greens have demonstrated their commitment to preventing their use. Given the UK’s planning disaster, it’s not happening.