For the British, I don't think you can say it "collapsed". The empire fell apart, sure, but it's not like life for the British people turned into living hell; they still enjoy a very high standard of living, among the best in the world, surely better than the Americans.
By comparison, when Rome fell apart (which admittedly took some time), the standard of living was objectively much worse for average Romans. They went from a civilization with plumbing, toilets, cities, aqueducts, a Coliseum with water shows, and probably the highest standard of living in the world, and a society with specialization of labor, education, rule of law and a well-developed legal system, to feudalism, where they lived under feudal lords as uneducated serfs living in huts and working in fields. It took over 1000 years for western society to get back to the level of technology and development they had under the Romans.
By comparison, when Rome fell apart (which admittedly took some time), the standard of living was objectively much worse for average Romans. They went from a civilization with plumbing, toilets, cities, aqueducts, a Coliseum with water shows, and probably the highest standard of living in the world, and a society with specialization of labor, education, rule of law and a well-developed legal system, to feudalism, where they lived under feudal lords as uneducated serfs living in huts and working in fields. It took over 1000 years for western society to get back to the level of technology and development they had under the Romans.