There is some evidence, yes, but it is not always a direct one. Just forget about pugio and pilum. When the Roman forces left Britain, that been a long time those weapons had been left over.
Current view among most scholars is no longer that Britain returned to its "celtic past" in 410 (or whatever date the Romans left, if they did really left in a brutal way). We know 5th and 6th century Britain had a real romano-christian culture, with an elaborate latin litteracy and a tendance to conservatism. What you will nowadays call Cornwall, Devon and Wales of course was less romanised than eastern Britain or the Severn estuary, but even those regions show an adoption of this culture, and even traded with Byzantium.
What is true with religion and other aspects of culture should be true aswell for the military. Of course Briton petty kings and tyrants had probably neither the organisation nor the manpower to field legion-likes armies, yet it is possible they sticked for sometimes to roman military tradition - mixed up with "celtic" and germanic warrior traditions.
The very fact they invitate barbarian federates to fight for them - the beginning of the saxon "conquest" show the late roman tradition of hiring
foederati.
Some Briton kings used roman military titles: most famous is Voteporix,
protector of Dyfed. Such men would have tryed to kept roman military inheritage (or at least what they thought as it). Same happened in other parts of the dying western empire, such as Gaul or North Africa.
Belt buckles are generally associated with the military. We do find a lot of buckles in Britain, with regional specificities, in the 5th century. We also see some late roman buckles beeing developped into "anglo-saxon" buckles in the 6th century. In the 7th century, we see an East Anglian king probably trying to look like (at least in part) a roman military leader - that is the famous Sutton Hoo king.
We know some brittonic words like
lluric in the poem
Y Gododdin beeing adoptions of roman words. In this case,
lluric stands for
lorica, ie armor.
7th and 8th century anglo-saxon helmets are ridge helmets, just like late roman helmets. Looking at the way they are built, it is fair to assume they are derived from late roman helmets. It is highly possible the Britons kept roman standart of weaponry, and eventually passed it to the Anglo-saxons.
We also know the famous late roman
draco had a long use in Britain, and is possibly the origin of the modern welsh flag.
There is possibly a crossbow depicted on a Pictish stone, showing the survicance of this weapon quite late. And a crossbow bolt was found in South Cadbury excavations, in sub-roman context.
In Gaul, it is possible that Britons served in the "last legions" which survived the fall of Rome, and in the middle of the 6th century were incorporated in the armies of the Franks and the Armorican
civitates. At least that is what Procopius is saying, and that is also the focus of my re-enactment group :wink:
You are wondering about bows: I fear there is no evidence until much later. But even in the 6th century we know that Britons were quite akin to ambush and guerilla warfare. Gildas told us people hide in the forests to fight off the Saxons. The 7th century byzantine military treaty called the
Strategykon even refers to the Britons as beeing good at fighting in woods.
Edit: I almost forget to throw this in the discussion.
This is a picture from the 5th/6th century codex called the
Vergilius Romanus. There is much debate about its geographical origin, but for some scholars like Ken Dark, it is possibly of British origin. If this is true, then you have here contemporary depictions of sub-roman briton soldiers, much similar in their equipement with late roman soldiers - of course with a
bit of artistic licence!