I dumped VM many years ago when they indiscriminately started throttling web services (I couldn't even load a web-page) and I noticed price increases on my bills without any accompanying documentation to explain why.
I went to TalkTalk, and then went to Sky when my TalkTalk contract expired. The only issue I have experienced with Sky was when my modem died, so a couple of days without 'net connection. Sky also diagnosed an issue with my girlfriend's connection at her family's home within 5 minutes that TalkTalk took 6 weeks to fail to rectify (fixed within 3 days by Sky - required an Open-Reach engineer).
That said, YouTube - like BBC iPlayer - is hit and miss in terms of buffering. I am a fully signed-up member of the Jobsian cult (iPhone, iPad and MacBook) and both have always had issues on iPad. The oddity is that I can stream Yumi Kamaza's 'Slave Wife' (definitely NSFW for me to Google confirm) but I do experience periodic buffering issues with YouTube.