When it comes to trade and Brito-Romans of the Vth century, there's unfortunately less information on the commercial exchanges with mainland than we have for later periods or Anglo-Saxons, but there's still some paths you could follow.

British economy in the Vth century was probably as it was before largely agrarian, both with cultivation and cattle. What changed was that the diocese was increasingly peripherialized, isolated and eventually cut off from the Late Roman Empire and its successor states : the injection of coinage (trough a top-down transmission from imperial salaries and subsides to local economy that ended up being taxed) declined since the late Vth century and stopped with the Vth.

Production surplus couldn't be exchanged anymore for coins or high economic goods; and was either redistributed or wasted in a regional economy that became increasingly localized both in production and exchanges; possibly followed by a production decline without motivation to produce an huge surplus for the Empire or empire-related business in Britain, especially as intensive farming and villae disappeared along the urban structures of an exhausted Roman Britain, unable to support them any longer.

Similarly, workmanship didn't disappeared but had to adapt to this situation, making different commodities for a, possibly purposefully, downsized production and economy already perceptible in the IVth and accelerated up to its historical outcome due to the Roman state collapse in Western Romania.

This new economic situation meant different needs (displaying a "militarized" and "ostentatory" Romanity), different access (although either repurposed towns or villaes, or hill-forts were close to the old pathways, exchange might have happened along kin or patron/client relations) but not different expectations : Mediterranean goods still came to Britain from the Eastern Empire trough Frankish Gaul or, bypassing it, around Spain until the late VIth century.

This trade, however, was not taking place in a market-economy, but part of a diplomatic-economy where the Eastern Roman Empire made a great display of its power trough gifts of coins, silk, oil, wine, etc. and either directly or indirectly, products made their way to provincial courts, and likewise redistributed. Western Britain, on this regard, gave what remained of the habits of surproduction of grain and leather, but possibly tin,furs and slaves as well.
Not that it was a monopoly of the local king or elites, and that exchanges passed trough their own domestic economy necessarily; but they probably presided to the exchanges, at least trough merchants that were part of their household.

These exchanges happened , probably not at their own halls but close to new centers as "fairs" of sorts, described as such by Richard Hodges

[...] held on boundaries, mostly in fact on the coast. Foreign traders probably visited these for short periods annually or perhaps seasonally. [...] Perpaps we can find some functional similarity in the Icelandic medieval fairs.

Rather than docks or purposefully built ports, which appeared later on in the North Sea Basin (Ispwich, for instance), we'd be more looking at natural harbors and landing sites in shallow waters, maybe already the place of regional or local exchanges. There's no real systematical outlook to these, and there wouldn't be anything wrong depicting these exchanges taking place in a remote location, a farm or set of rural buildings, a coastal village or in a repurposed or semi-abandoned villa.

Ships involved were probably of the kind that existed in the Mediterranean Sea, flat-bottomed vessels (possibly mixed with "glass-wine" careens) and a lateen sail. Something comparable to this (Kelenderis Mosaic), best suited for the lack of docks or infrastructures.

These fairs were in themselves displays of power and "interegional/international" consideration of Brito-Roman elites, in the words of Robin Fleming

[it] allowed the most important members of the community to reassert their romanitas and to underscore their superior position within the society of the rebuilt hillfort

Similarly, carrying goods from the Channel harbors to the royal or elite residences might not have been a mere logistic happenstance, top-down redistribution being a political and cultural tool biding communities and alliances.

As such, you could represent this diplomatic trade fair as an important event of this "arthurian" community with not just traders but envoys or spectators.

- Britain after Rome, the Fall and Rise 400 to 1070; Robin Fleming; 2010

- Dark Age Economics, the origins of towns and trade AD 600-1000; Richard Hodges, 1982 (reed. 2012)

- L'adaptation des navires aux espaces de navigation durant l'Antiquité Tardive en Méditerranée; Émilien Afane; 2012

- The Ruin of Roman Britain, an Archeological Perspective; James Gerrard; 2013

- Wales and the Britons 350-1064; T.M Charles-Edwards; 2013

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