The Normans took over but they just affected the stuff at the top. Most of us remained as we were and yes I know we had all kinds of Viking and other invasions. The ordinary people, most people, were not hugely affected by the Normans or the Romans.
They were massively affected. Our political and legal systems were established by the Normans, and Roman influence on culture, engineering and architecture was colossal.
There is no such thing as a pure British race or culture, and nor will their ever be.
Ecological degradation is only one consequence of market capitalism. As the economy is digitalised and robotics advances, huge areas of employment will disappear. Increasing numbers of human beings will become economically unnecessary. Economic and social inequality could revert to levels last seen in the 18th century. The capacity to 'edit' genes already exists - hitting the headlines last week actually. Governments will try to regulate it, but in vain I fear. Designer babies will become available to the rich, allowing them to perpetuate a genetically 'superior' elite. Genetics will become another form of capital. Joining financial capital, social capital, cultural capital and erotic capital will be genetic capital. 'Good breeding' once meant belonging to a feudal dynasty; in the future it will be available on the market - beautiful, intelligent children free of disease. At the other end of the spectrum, a precariat of genetically 'inferior' workers will probably struggle to eke out a living service sector industries or the sex industry - though even their jobs will probably be threatened by sex robots.
The ice caps will melt and geopolitical conflict will ratchet up. India and China will be the ascendant economies, and the US and developed Europe relegated to second world status. Terrorism and cyber-terrorism will come be completely beyond the control of governments. Social problems such as crime, mental illness, suicide and homelessness will reach hitherto unknown levels.
As societies go into meltdown, extremist religious and political cults like ISIS and those of the neo-fascist far right will gain power.
This sounds like a sci-fi dystopia, but I cannot imagine any other prospect at the moment.