The actual evidence against the Tudors being I1 is a lot fuzzier.
At the outset you have to give up the idea that there were I1 people living on the glaciers that covered Britain. I1 didn't exist yet. Timing is everything.
If you give any credit to all the old genealogies, then the Tudors would be direct male-line descendants of Beli Mawr, a legendary king of Britain 100-200 years before the Romans arrived. He's the ancestor, one way or another, of all the Welsh royal lines. Problem is, he's really just the old Welsh sun god turned into a human ancestor by later Christians.
Somewhere along the line between the Tudors and Beli Mawr there is probably some invention. Maybe a lot of invention, but no one is fully sure where it was, so you have to take the problem in stages.
The earliest totally certain Tudor ancestor was Ednyfed Fychan (13th century). As far as anyone can tell, all of his male line descendants are extinct. There is, however, some backroom chatter that there is guy out there somewhere who claims to have proved his line back to Ednyfed Fychan. Some people say he is R1b and is getting ready to publish his proof.
Ednyfed Fychan was supposedly a descendant of Marchudd ap Cynan (10th century). I say supposedly because this isn't my area. I simply don't know if that's likely to be true, but it's only 200 years and it seems plausible to me that the tribal Welsh could have preserved a pedigree that long. Supposedly -- again, outside my area so I don't really know -- there are hundreds of men living today who can trace their ancestry in the direct male line to Marchudd -- and supposedly almost all of them belong to a closely related group within R1b. The only ones I've seen mentioned by name are the Williams-Bulkeley Baronets and the Barons Mostyn. Some people say they are both R1b, but I haven't found any proof.
Marchudd ap Cynan was supposedly a descendant of Arthwys ap Mor (5th century) who was a king in what is now northern England. This Arthwys was supposedly a descendant of Beli Mawr.
At each stage further back, the ancestry get more improbable. I haven't paid much attention. I've exchanged messages with people who claim they prove a direct male line to these folks, but I'm a bit skeptical. The few who've told me about DNA have all said they're R1b, which to them is part of the proof that their claim is true.
So where are the I1s? I'm just not seeing them.