How thick was the ice?
How do you meaasure the thickness of a glacier? For existing glaciers, we have a number of useful tools to do this - such as ice-penetrating radar which can see through the ice, or drilling through a glacier to the rock below. However, these methods do not work on glacier that no longer exist - as is the case for the British Ice Sheet! In fact, measuring the size of a piece of ice which melted away 20,000 years ago sounds rather impossible. We do have a few different techniques that we can use to estimate how thick the ice might have been:
1) Using the thickness of other existing glacier today. Glaciers of the past were fundamentally made up of the same things as today's glaciers, and so we would expect their thicknesses to be fairly similar on average for a comparable size. When no other data is available, we can use this to roughly estimate the ice thickness. The map below shows the ice thickness of Antarctica (from Bedmap2; Fretweel et al., 2013) - ranging from a few hundred metres to four kilometres. The British Ice Sheet was quite a bit smaller than Antarctica is today, so may have mostly included the thinner ice.
2) Using computer models of glaciers. We understand the physics of how glaciers flow fairly well, and can use a computer to understand what the glaciers may have looked like over Scotland 20,000 years ago. The image below shows an example of this from Kuchar et al., 2012.
3) Using available field data. Glaciers are excellent at leaving evidence of how wide they are, dumping large piles of rock and sand at their terminuses. They are not so good however at recording their height - indeed many mountains are just buried beneath the ice entirely. Some rare peaks may be exposed, which can help calculate the maximum ice surface elevation in the past. We can also get indirect data on the ice volume by measuring changes in sea level elsewhere in the world - as the enormous amount of ice melts it pours into the ocean and raises the water level globally.
In summary, we have a few methods which we can use to calculate how thick glaciers may have been in the past. In most cases these are not extremely accurate - so many questions about past glaciers are still to be answered!