The picture of high tides on opposite sides of the Earth with a period of about 12 hours (actually 12 hours 25 minutes, due to the rotation of the Earth) is an oversimplification. It's just a starting point.  Tides would behave this way in the limit of an all-water Earth with ocean depth so great that it had no effect on the surface wave. 

But the Earth has continents , peninsulas, bays, estuaries and the like, and the ocean has a finite depth causing frictional effects on ocean waves, and characteristic frequencies of the ocean basins. All of these factors, plus the Coriolis effect due to the Earth's rotation affect the boundary conditions of the variation in ocean height due to tides. In turn, depending on local coastal geography, local basins can have characteristic resonant frequencies leading to local constructive or destructive interference with the tides.

All of these effects lead to higher order harmonics in the tides on top of the 12 hour 25 minute primary tide. By higher order, I mean that these components of the tides have higher frequencies (shorter periods). And they can be locally important.

It's these short period effects (periods of a few hours, not 12+ ) that would explain what's going on in locations such as the two places in England.