Britain was a last minute entry into the Roman Empire. It had been ‘invaded’ by Julius Caesar twice during his Gaul Campaign, his stated reason was to carry the Aquila to the end of the world and to punish the British for giving aid to the Gauls. It’s doubtful whether the British did anything in a formal way, hardly ‘aiding the enemy’ as we understand the term. Both invasions were fiascos for Caesar and ended almost as quickly as they began. Though Caesar did manage to install a friendly King over the Trinovantes. Caligula assembled 200,000 men (so he claimed) to invade the island then had his men collect sea shells instead. It took Claudius to bring Britain under Roman rule and he did it to prove his ability to rule the Empire during peace or war. Boudica’s revolt was the last native revolt we hear of within Britain proper for years, but there must have been a few as a legion was always kept within the province for years after Boudica.
But from the very start Britain was a pain in the backside of every Emperor onward. So remote and hard to reach Britain became a perfect starting point for many a pretender. Hadrian and Antonius Pius both had walls built, partly to define the end of the empire, to defend the province against the northern tribes living in what today is Scotland, and to give the legions something to do besides revolting. The last may have been the most important benefit as far as the emperors could see.
And for all, that Britain was profitable to the Empire for at least two centuries. Hadrian’s wall shows it was a magnet for trade, many a village, or town was created along it’s length by a mixing of retired legionaries and natives. Grave markers show merchants and their family migrated to the fog bound island to trade from as far away as Syria.
When the last legion left Britain to defend the Rhine against German tribes Britain was still nominally a Roman Province. Never-the-less the empire never returned. A series of petty tribal chiefs, know as ‘Kinglets’, ruled in the name of Rome. It is not easy to know when Britain finally broke free and it’s leaders rule in their own name, but surely the loss of Britain was not a great hardship to the Emperor’s. The locals however took a great deal of convincing, caches of coins treasures and goods have been found; these are thought to be hordes held for the day the legions and their Emperor returned. Some of these caches are almost 100 AFTER the empire had left.