Sea level

This fluctuating red line indicates the relative height of the sea level, which has been gauged from marine cores. ‘0’ is indicative of present day sea level, whilst ‘-120’ denotes a sea level believed to have been, on average, 120 m lower than today’s. This lowered sea level was a consequence of ice age conditions. As temperatures dropped water was locked into ice sheets, predominantly at the poles though at times they extended so far south that parts of present day Scotland were under ice. 
On the chart, you will notice points where the sea level was higher than it is today (labelled 11, 9 and 5e). These indicate warm periods with balmy temperatures and higher sea-levels. During these periods, animals better suited to hot climates stalked London and Kent and the remains of elephants, rhinoceros and lions dating to 420 thousand years ago (11 on the timeline) have been found. 125 thousand years ago (5e on the timeline) hippos swam in what was to become the Thames and lions roamed Trafalgar square.
1985 Sea level change mapping. Based on Coles.
The numbers on the red line represent marine isotope stages (mis), also known as oxygen isotope stages. They symbolise alternating hot/cold periods. On this simplified timeline they show the dominant stages that have affected humans and the human occupation of Britain over the last 1 million years. The full list of marine isotope stages extends back to MIS 104, over 2.5 million years ago. The data for these has been derived from marine cores and the measurement of levels of oxygen-18 within them. The alternating pattern means that odd numbers denote warm periods whilst even numbers correlate with colder periods.
The numbers on the left (0 -30 etc) An approximation of sea level, given in metres and typically in negative numbers indicating a sea level lower than today’s, is given on the left hand side of the diagram. ‘0’ indicates today’s sea level. This is a simplified version but serves as a good illustration of successive marine transgressions and regressions. 
Warm/cool – Whilst these labels cannot convey the arctic blasts and frozen plains of a glacial maximum, nor the balmy swamps of interglacials, they do give some approximation of temperature and general conditions and should be read by the height of the red line, which also denotes relative sea-level.
18 dO ‰ – The scale on the right of the diagram (which ranges from 3-5) indicates the differential change in the levels of oxygen-18 recorded from marine cores. This is the information on which the timeline of glacial change is based. 
KA – This denotes 1000 years and in this context refers to 1000 years before present (also transcribed kya – thousand years ago, or KA BP – thousand years before present). The scale on this timeline extends back to 1 million years ago (1000 kya) with the earliest evidence of human occupation in Britain appearing around 900,000 years ago (900 kya). This date has moved back significantly in the last two decades because of discoveries from the east coast made at Pakefield and Happisburgh.