On the occasions that I've had to call Virgin Media with problems, and I've had an offshore call centre, the issues have been resolved satisfactorily, both with faults or changes to my services
One can encounter difficulties with accents, but then as a southerner, sometime a Scottish or other northern accent it it's strong can throw you.
I suspect the greatest difficulty and frustration is that offshore will adhere quite rigidly to any procedure scripts shown on their screen, whereas a UK agent might be willing to deviate especially if the customer can offer some fault diagnosis of their own. But you can't really criticise someone who follows procedure.
Of course there are too many times when things don't go to plan. We've seen plenty of examples of that over the years on Cable Forum. But for all the posts over which call centre customers speak to, it isn't that clear, at least on here, whether UK or offshore centres actually do have a higher failed resolution rate, or if it is just a perception.