"Ostorius ... prepared to disarm the suspect and to overawe the whole district on this side of the Trisantonana and the Sabrina" (Annales XII.xxxi)
Chronologically, the first limes discussed by modern historians in Roman Britain is the so-called "Fosse Way frontier" (Webster tRIoB pp.159 & map p.121). This is based upon the Fosse Way Roman road (Margary #5) which linked two legionary fortresses and probably delimits the territory gained during the tenure of Aulus Plautius, the first governor of Britain. The classical premise for such a frontier is the passage from Tacitus quoted above which mentions two rivers, which have been identified as the Trent and the Severn; significantly, they are flanked on the south and east by the Fosse Way. Exeter at its starting-point in the south-west was very-likely the base of the Second Legion Augusta, as evidenced by pieces of Neronian roofing-tile (Britannia vii 1976 pp.278 & pp.358), and Lincoln at the north-eastern terminus was occupied by the Ninth Legion Hispana, attested by the tombstones of its soldiers (RIB 257, Burn 6, et al.). This military highway was punctuated by smaller auxiliary stations such as Margidunum/Castle Hill in Nottinghamshire, a fortified settlement astride the road which has a known Claudian foundation (Collingwood pp.65/6; Frere pp.79).
The signal-stations on the north Devonshire coast at Martinhoe and Old Burrow overlooking the Silurian lands across the Bristol Channel seem also to have been part of this early occupation phase, the former yielding coins of Nero, the latter those of Tiberius and Claudius (Dudley & Webster, p.151).
During the rule of Claudius the Romans were engaged in warfare on three continents and were still claiming new territory. The two Mauretanian provinces were conquered by Plautius in 42, where his exploits had earned him the respect of the new emperor, who promptly set him the task of conquering Britain. There was also fighting against the Germanic tribes, notably the Chauci and the Chatti, in northern and southern Germany respectively. There was Claudian development also along the Danube; auxiliary forts being built in Moesia to quell the recently conquered tribes and Noricum was annexed in 46. There was also serious unrest in the east, from the Crimea to Syria, most of which had been dealt with by diplomacy (Levick, Chap.14). This period of active warfare was not conducive to the development of frontiers, with territory still being claimed, both in Britain and on the Continent, any such systems would have been extremely transient. Indeed, the "Fosse Way frontier" itself, was to last only until the second governor of Britain, Ostorius Scapula arrived in 47, who immediately abandoned it to campaign against tribes in North Wales (Annales XII.xxxii).
The idea that the Fosse Way constituted a Roman "frontier" in any real sense has since been discredited, and it was not until the coming of Gnaeus Julius Agricola that Britain was endowed with the first planned frontier works.