Auxilia Cohorts and the Claudian Invasion of Britain

[Centurio]

A quote from G. Webster. The Roman Invasion of Britain. London, 1993, pp. 86-87:

The auxiliary units
"The identification of auxiliary units is a more difficult matter, since the names of units are very rarely given by the ancient historians. One has to rely on inscriptions, of which there are very few of this early period. It is possible to draw up extensive lists of units in Britain during the second century from the discharge certificates, known as diplomata,in the form of bronze plates,23 but the earliest one from Britain is dated to AD 98, and by then many changes in unit dispositions had taken place.
Assuming that the auxiliary strength was kept in reasonable balance with that of the legions, there may have been at least 40 units of quingenary size (i.e. 500 men) allocated for the garrison of Britain, but in the early years of the conquest there were probably more. Fortunately, one can make an excellent start in identifying particular units if one accepts Mark Hassall’s well-considered argument for eight Batavian cohorts being included in the initial force. The later attachments of these units to XIV Gem suggests that they may have accompanied this legion from the Rhineland, and as will be seen presently, made the vital breakthrough in the great two-day battle of the Medway, by forcing a crossing of the river, as they were trained to do. The evidence from the tombstones is limited to the monuments from Colchester, (RIB 201) and Gloucester (RIB 121), both are Thracian units, Ala I Thracum and Coh VI Thracum respectively, since both appear to belong to an early phase of the occupation. The use of these Thracian troops at the invasion period may seem strange, in view of the serious trouble in Thrace in AD 26 when some of the tribesmen revolted against the introduction of the military levy. Their main grievances were that their clan system would be disrupted, with the young men forced to serve in distant places far away from home. Seventeen years after Poppaeus Sabinus had so crushingly reduced the rebels, regular Thracian troops were part of the Roman army for some years. Longinus, the duplicarus at Colchester, had served 15 years at the time of his death, which must have been a year or so after AD 43. An auxiliary of a Thracian cohort also died at Wroxeter (RIB 291) presumably between c. AD 50 and 56, but the number of the unit has been lost in damage to the edge of the stone, so it could have been the Gloucester unit moved from here when Legio XX was transferred from Colchester in AD 48.
The only other possible invasion units are those on the two stones at Cirencester, and one at Bath. The former two record the presence of the Ala Indiana and Ala Thracum (RIB 108 and 109), either brigaded together as a large mobile task-force, or one succeeding the other, the Thracian unit could be that from Colchester, moved forward by Scapula. The Bath stone (RIB 159) records a serving trooper of the Ala Vettonum, the unit later to occupy Brecon Gaer in Wales, but the granting of citizenship signified by the letters CR (civium Romanorum) after the man’s name seems to show that this stone cannot be earlier than Vespasian."

Hope it's helpful.