After the Romans left the UK, a lot of their tech stayed around. There were still working hypocaust, the roads were still there, the cultural melding ...

But some technologies were lost. How extreme or dramatic this was seems to be a matter of debate. 

I wonder if there were Saxon age brits who were moon-eyed for Roman technology, who kept trying to do things the way the invaders did it. I wonder if other Saxons rolled their eyes at them.