To the OP: Yes, of course Britain can be invaded, especially if the whole world was against it. It comes down to numbers. Simply, Britain has about 70 million people. The rest of the world has close to 7 billion (that's 7,000 million). Not to mention the extreme imbalance in industrial capacity between Britain and the rest of the world.
Plus, the US Navy alone could easily wipe out the Royal Navy, and its air force could do the same to the RAF. The invasion bit would be a bit tricky, granted, but it's nothing that massive aerial and naval bombardment can't clear the way for.
In addition, an all-out war would target not just military, but infrastructure targets as well. Rails, roads, major highways would all be subject to massive bombardment. This would lead to food shortages in major cities, as food from the countryside farms would not be able to reach population centers as easily. As well, raw resources for factories would be difficult to actually get to the factories.
Eventually, the combined pressures would simply wear down the country, making it relatively simple for the tens of millions of soldiers of foreign countries to walk in and mop up.
Plus, the US Navy alone could easily wipe out the Royal Navy, and its air force could do the same to the RAF. The invasion bit would be a bit tricky, granted, but it's nothing that massive aerial and naval bombardment can't clear the way for.
In addition, an all-out war would target not just military, but infrastructure targets as well. Rails, roads, major highways would all be subject to massive bombardment. This would lead to food shortages in major cities, as food from the countryside farms would not be able to reach population centers as easily. As well, raw resources for factories would be difficult to actually get to the factories.
Eventually, the combined pressures would simply wear down the country, making it relatively simple for the tens of millions of soldiers of foreign countries to walk in and mop up.