On Monday 27th June 2011 at about 10.30 am the south coast of Cornwall was hit by a small tsunami. This was a wave of about 20 cm when it hit Newlyn, but as it moved along the south coast it grew, reaching 30 cm at Plymouth and 40 cm by the time it had reached Portsmouth it had grown to 40 cm. In enclosed estuaries it grew further, reaching 70 cm in the Yealm Estuary (near Plymouth).
Tsunamis grow as they enter enclosed spaces such as estuaries or the English Channel (the growth of this particular tsunami from west to east occurred as it moved into the English Channel) since they are essentially pressure waves. Some event causes a rapid change in seawater pressure (typically an earthquake or a submarine landslide), which spreads out like ripples on a pond after a stone is dropped in (which is a small pressure wave). On open water this eventually dissipates, but if it reaches a coast then things get interesting. The sea narrows close to the shore (obviously) so when a wave of water hits it, it is forced up above the surface; the shallower the water, the higher the wave. Particularly large and powerful waves will continue over the land from the coast for some distance.
Of course waves do not just go up, they go down. So a tsunami isn't just a wave above the normal sea-level, it is typically preceded and followed by rapidly shallowing water. Thus people in areas prone to tsunamis know to evacuate the coast rapidly if the tide apparently goes out rapidly, since this is likely to be followed by the tide coming in rapidly. This gives us the English term 'tidal wave' which is no longer used, since it is inaccurate; the tides are caused by the gravity of the sun and moon, tsunamis are nothing to do with the tide. The term 'tsunami' comes from Japan, where earthquakes, and therefore tsunamis, are all to common.
The presence of a tsunami implies that an earthquake or landslip has happened. No earthquake was detected by seismometers in the UK; this is not greatly surprising as we don't have many. In the absence of any data, the most likely cause would be a landslip about 400 km of the coast of Land's End, where the continental shelf ends abruptly and there are steep marine cliffs. However landslips here usually cause small tsunamis on both coasts of Cornwall, not just the south. This leaves the possibility of an offshore earthquake somewhere to the south of Devon and Cornwall. This would be the third earthquake in the area within a month; a small earthquake struck 20 km east-south-east of Dartmouth, Devon, on the second of June and a small onshore earthquake hit the town of Newton Abbott, again in Devon, on the 23rd. These events could possibly be related; some sort of adjustment in the structure of the Earth beneath Devon and the western English Channel, possibly related to the expansion of the Atlantic, or the movement of Africa (pushing north into southern Europe) or Spain (rotating anti-clockwise).