It seems strange to me that the Romans dedicated so much to the invasion of Britain, especially after so much loss.
What was the economic benefit? What did Britain have that was worth crossing the channel for?
Which time? Caesar or Claudius?
If you are thinking about Caesar, mostly because he need conquest to support his political campaign, against pompeius victories in the east, and to gain support from his men by winning and also because under marius military reforms the republic need land to give to retire soldiers as a form of "retirement pension". And also because it was a farily easy target with little divided-not organized-tribes, oftenly primitive, in comparisson to west germany tribes.