Hey.

Couple things on this.
We definitely DO have manica finds in Britain, but they were dated to the 3rd or 4th century, if I recall right. About a hundred or two hundred years before the period I am looking at, but they have been confirmed to have been fighting armor at that time. Actually the find was a rather big deal, it was the first of its kind on the island I guess (about a year ago, will dig for the link later). It is worth noting that the protection offered by manica is unclear; we have evidence some protected the hand, but whether that was universal, or an earlier period )1st/2nd centuries CE) is unknown at my last reading.
There were also rare finds of splinted greaves and vambraces all across the northern regions of the Western Empire for this period. Stephenson suggests (in "Romano-Byzantine Infantry Equipment") that the Valsgarde greaves may have been influenced by Roman infantry gear. The ones I am thinking of were made of seven metal splints, all riveted to straps side by side (sides touching or close). Not your standard "14th C splinted greave", with more leather than greave - quite the opposite.
I found a link to a reference for Anglo-Saxon greaves as well (
http://tinyurl.com/m98qw9) but have not verified the reference. However, it does claim that these greaves were rare, although multiple sources suggest both metal and leather greaves were used. It seems highly likely that if the Saxons were using them, the Romano-Brits fighting them might well have also; we know that much transference of other culture (including clothing and jewelry styles) did occur - although in this case it seems more likely that the greave use flowed from Roman to Saxon, rather than the reverse (this seems common - military tech flowing from Roman to barbarian, and culture flowing from barbarian to Roman).
During the 4th-6th centuries (til the middle of the sixth or so) there was still a well organized Roman style government in Brittania. It's arguable that the last Western Roman government was actually in Brittania; things held together much better there in general than they did on the continent where migration pressures were much stronger. Two things can be drawn from this.
First, we know that regular trade continued with the continent - Gaul, and Rome, and from there to beyond - through these centuries. While that trade had periods where it was reduced due to calamity on one end or the other, it seems to have never been completely stopped during this time. That means customs, styles, and materials would have progressed (slowly) from continental armies to the islands.
Second, it's important to remember that the government of the late 5th and early 6th in Brittania saw itself as Roman. They used Roman titles for civil and military leaders; they often used Roman names. The era was one of attempting to fit the old systems into a new time (and ultimately failing, but lasting longer than most). It seems likely therefore that an attempt would have been made to retain 'modern' Roman armoring customs as much as possible. Trade still existed; industry was still largely intact in the western island; and you had a culture largely dominated by military leaders.
Armor of any kind on the island was a sign of wealth, however. Most common people arming themselves to fight would have worn a subarmalis or thoromachus (think gambeson, sometimes made of a single thick felted layer instead of layered cloth), possibly with a leather shirt over it. A shield and spear would have rounded out the ensemble, perhaps with a felt cap.
Professional soldiers of the era were likely very different, however. The Romano-Britons used cavalry extensively; organized and permanent cavalry units implies a level of centralized control of wealth that infantry does not (feeding, keeping, training horses is an expensive process). It is likely that these troops had mail or scale, possibly leg greaves, shields with iron bosses, javelins, spears, swords, helms, and perhaps forearm and shoulder protection of some sort - much like their heavy cavalry counterparts on the continent.