The Roman general Carausius set up a breakaway state in Britain and part of northern Gaul, independent from the main Roman empire, in 286; he was murdered and succeeded by Allectus, who ruled from 293 until the main empire reconquered Britain in 296. The lack of complaint within Britain at the usurpation of power by Carausius and Allectus gave some sign of the shape of things to come, and suggests that Britain's interdependence with Rome was already weakening.

Diocletian tried to check inflation by replacing the debased coinage with real silver and establishing a Price Edict mandating price levels. Unfortunately, the Empire's fundamental economic problems went unsolved, as the high taxation required to fund the military fueled inflation, the Price Edict was unenforceable, and output continued to decline.

Constantine the Great, who was Emperor from 307 AD, had strong ties to  Britain and was with his father Constantius at York when the latter died in 306. In Britain as elsewhere, he strongly encouraged the improvement of roads and fortifications.

Christianity had been introduced into Britain before Constantine. The first British martyr - St. Alban - was executed some time in the 3rd or early 4th centuries, traditionally in Diocletian's time, but possibly earlier. [Verulamium was later renamed St Albans in his honor.]

Constantine died in 337, but in 342 his son and successor Constans visited - probably to help organize a response to Pictish raids.

Britain fell under the control of Flavius Magnus Magnentius (c. 303 - 353) who usurped the imperial throne in the west in 350. It is possible that troops from Britain were among the 50,000 killed at the bloody Battle of Mursa (351) when Constantius (another son of Constantine) defeated Magnentius.

During the mid 4th century, both Scottish and Pictish tribes attacked the frontiers of Roman Britain. The most destructive invasion was the coordinated assault of 367-8. It was suppressed by a force sent from Gaul under the military commander Theodosius (father of Emperor Theodosius, who ruled from 379-395). However, there were further attacks in 382 and in 396-8.
 

Invasion and high taxation severely disrupted trade, and British cities declined as centers of industry and commerce. Yet as late as 400, the situation was far from chaotic as the countryside suffered less disruption, and Theodosius had constructed a series of fortified signal stations along Britain's coasts to provide advance warning of seaborne raiders.

 

The 70-foot Nydam Ship:  an example of the type used by coastal raiders of the 4th century

On the death in 395 of the Emperor Theodosius, the Empire was divided  between his two sons - Arcadius receiving the West and Honorius the East. The Eastern part of the Empire had long been growing relatively more powerful and prosperous. In the West, imperial power fell increasingly into the hands of military commanders, often of barbarian descent.

Britain's coasts were under attack by Germanic raiders all along the "Saxon Coast" of South and East England. But by 401 the Roman homelands themselves were under attack, and Flavius Stilicho, the effective ruler of the Empire, withdrew most of the legions garrisoned in Britain.

The British did try to "defend themselves" - or at least to hire one set of barbarians to fight others. Bede's history - written long after the events - stated that Vortigern, a Romano-British chieftain, hired the Jutish leaders Hengist and Horsa to help repel the Scots and Picts. These mercenaries then turned on their employers in about 446 and seized South East England for themselves.
The individual characters may be merely legendary, but that Germanic invaders streamed into Britain and created their own kingdoms is clear.

Londinium which in the third century had boasted a walled area of 330 acres, had decayed by 430. It fell into ruin, and would not prosper again until the middle of the seventh century.

Roman organization and culture had disappeared by about 600.

Four centuries of occupation left their mark on the British landscape. The network of roads running arrow-straight through the British countryside marked routes that survive to this day.
Although Roman cities were decayed, many expanded again later, and some (such as Canterbury, and Dorchester on Thames) may have been continuously occupied, though not as real urban centers.

The Anglo-Saxons were pagans, but British Christians who fled to Wales and Ireland retained their beliefs.