The derestricted stretches of Autobahn are becoming more scarce. Most stretches actually already have speed limits - some of them quite low by UK standards. Many urban stretches, or stretches close to junctions etc. are limited to 100km/h (~60mph) or less. Some stretches which pass close to cities are restricted to even lower speeds due to 'L�rmschutz' (noise abatement).
There is a country-wide 'advisory limit' of 130kph (~80mph). If you are involved in an accident on derestricted motorway and you are travelling faster than 130kph, you will automatically legally be held partially responsible.
I would suggest that the more important difference between the UK and Germany is the standard of driving. Anyone driving in Germany will notice that in general drivers pay close attention to speed limits (even without the use of cameras), do not tailgate and have greater lane discipline. Part of the reason may be that the Polizei enforce traffic laws with great vigour!