I would believe that I-Y6910 originated somewhere in the Northern Germany-Jutland area. If I recall correctly it is downstream of I-A196. I would like some ancient DNA to be Z140 positive.
Your I-Y6910* line is not positive for the downstream SNPs that traveled to Britain & Ireland, while you are related to those people you are not related for a very long time.
Is it possible? Maybe. So far it isn't looking to be likely. Both R-U106 samples found at Driffield Terrace were autosomally some concoction of Germanic (one of them appeared to be Celto-Germanic). One of them was positive for the defining House of Wettin SNP, many Roman auxiliaries settled down in Roman Britain with their families. We must also remember that there were a large number of Germanic foederati troops in Roman Britain coming from tribes such as Tungri, Frisiavones, Batavi, and other groups. I also think it is worth mentioning the Marcomannic chieftain Fraomar who, with his followers, was shipped off to Britain after he is stripped of his chieftainship.
It wouldn't surprise me if there were some Anglo-Saxon foederati in Britain in these days. The coming of the Anglo-Saxons doesn't appear to be this full-scale invasion as was previously thought.
In regards to some Celts and Germans being indistinguishable are you referring to the Y-line (which is such a small portion of ancestry) or do you mean autosomal? Both the Celts and Germanic peoples have similar origin populations in varying proportions (reflected in autosomal DNA) and are rather similar if we don't get down to the microscopic differences.