To reconstruct Britain of millions of years ago and understand how climate breakdown will affect the region, we can look at geological and paleontological records, as well as computer simulations. Here's a general overview of what Britain might have been like millions of years ago and how it could be affected by future climate breakdown:

Britain Millions of Years Ago:

- Tropical Rainforest: During the Eocene period, around 56-34 million years ago, Britain was located closer to the equator and had a warm, humid climate. Tropical rainforests covered much of the land, with lush vegetation and abundant plant life.

- Diverse Fauna: The Eocene epoch was known for its rich biodiversity. Britain was home to a variety of mammals, including early primates, carnivores, rodents, and even ancestors of modern ungulates like horses and rhinos.

- Sea Level Rise: Sea levels were significantly higher compared to today. The land area of Britain was smaller as much of it was covered by shallow seas and subtropical coastal environments.

Climate Breakdown and Potential Impacts:

1. Rising Temperatures: As global temperatures increase due to climate breakdown, Britain could experience warmer and drier conditions. This shift could lead to a transition from temperate vegetation to more drought-tolerant plant species.

2. Extinction Risk: Higher temperatures and changes in ecosystems could put pressure on native plant and animal species, potentially leading to biodiversity loss and extinction risks for species that are not well-adapted to the changing conditions.

3. Sea Level Rise Revisited: With accelerated ice melting and thermal expansion of the oceans, sea levels could rise significantly again. Coastal regions in Britain, including major cities like London, could face increased flooding, erosion, and land loss.

4. Habitat Changes: The expansion of urban areas, agriculture, and infrastructure to accommodate a growing population could further fragment and reduce natural habitats, putting additional stress on wildlife and contributing to biodiversity loss.

5. Changing Weather Patterns: Climate breakdown could lead to more extreme weather events such as heatwaves, droughts, storms, and heavy rainfall. These events could disrupt ecosystems, agriculture, infrastructure, and human communities, causing significant social and economic impacts.

6. Ocean Acidification: Increasing levels of carbon dioxide in the atmosphere can lead to ocean acidification. This process negatively affects marine life, particularly organisms that build calcium carbonate shells or skeletons, such as coral reefs and certain shellfish.

7. Altered Carbon Cycle: Changes in the carbon cycle could impact the amount of carbon dioxide and methane released into the atmosphere. Wetlands and peatlands, which are significant carbon sinks in Britain, could become sources of greenhouse gases if they are disturbed or dry out.

8. Feedback Mechanisms: Climate breakdown can trigger feedback mechanisms that further amplify the effects of global warming. For example, melting permafrost releases more methane, a potent greenhouse gas. Additionally, the reduction of reflective surfaces like ice and snow can lead to increased heat absorption, exacerbating temperature increases.

It's important to note that the specific impacts of climate breakdown on Britain and other regions will depend on various factors such as the extent and pace of global emissions reductions, local climate dynamics, and the effectiveness of adaptation measures implemented.