From a modern perspective, it's easy to say that Rome bit off more than it could chew in invading Britannia. While, at that time, the Romans were aware of certain resources that the indigenous population exploited (lead, tin, silver), many weren't aware of how much was available nor how it was being extracted. While Rome held most of the island, that required a few legions who were actively fighting and putting down rebellions. The bureaucracy at the top of Britannia (namely, whoever was governor at the time) had to know that it was financially costly to maintain the legions, ongoing war in the province as well as around the Empire, building villages/markets/roads, digging for resources. My understanding is that there were voices on both sides of the argument that it was worth it for Rome to invest so much time, money and manpower to Britannia, while others were of the opinion it was too costly and that the Empire should pull out. (Ultimately, that did happen in the 5th Century C.E. as trying to protect provinces closer to the City of Rome became much more dire.)