I don't think anyone will be able to give you an answer that will satisfy you.
To my knowledge, the earliest inhabitants of "England" would have been during the Neolithic Era (4000 - 2500 B.C.).
I'm not sure these people would qualify as a homogenous group of people, or that they were the "first" inhabitants of "England".
My information is that sometime around 4000 B.C. there was a migration of people from the Mediterranean to "England". They would have been farmers for the most part.
Following them were the Beaker people (2500 B.C.), so named from the shape of their pottery.
Both of these groups seem to have been involved in the construction of henges, including Stone Henges.
Following the Beaker People were the Celts, also who I'm not sure one could classify as a purely homogenous group.
Following the Celts were the Romans.
Following the Romans were the Angles and Saxons.
Following them were the Danes or Vikings, ending in the battle of Hastings in 1066. Which is where the Bretton origin of Forshaw is from: A knight named Fougere(sp?), (which is translated from French as Fern) fought in this battle and was bequeathed lands and title in England. Note: there is a mediaeval town in modern Brittany called Fougere.
I am no expert in Etymology (I only took it for a year) but I see no compelling evidence for the origin of the Forshaw name. My father used to say that we were fishermen and that Forshaw used to be Fore Shore - some oblique reference to low tide I guess. Anything is possible I guess.
I have found evidence of the name Forshaw in use as far back as the 1400's (around the time of the Was of the Roses), but nothing further back than that.
Modern English is comprised of words, grammar and meanings from possibly dozens of Indo-European languages - we may never know the "true" origin of the name, but that hasn't stopped me from looking or ruling out any possibilities.
As for a Welsh connection, I believe Manx is the variation on Celtic that is spoken in Wales and while I admit I have not had access to a comprehensive Manx/English dictionary, I have found no similar sounding words that would suggest an origin for us there. Neither have I been successful finding a documented genealogical reference to Forshaws in Wales (I am limited to the Internet as my main source of reference in this instance).
Interestingly enough, I recently had a discussion with a German friend on Etymology and the origin of surnames and he felt that Forshaw sounded German to him. Apparently the word Forshaw appears at the end of many German films meaning "The End" I guess. Unfortunately I have never seen the credits to any German films and I was quite drunk during our conversation. Again - a possible lead that I have yet to confirm or deny.