First / oldest Bell Beakers were
probably neither Germanic nor Italic nor Celtic because they were older than these groups emerged.
We could as well ask: were "Adam and Eve" Chinese, Germanic, Russian, Melanesian? No, they were too old to be any of those groups.
However, I think that by 900 BC (LBA) there were Celtic languages spoken in Scotland. That is just few centuries before Pytheas of Massalia visited Britain:
https://en.wikipedia.org/wiki/Pythea...ery_of_Britain
How do you know this
for sure? Linguistics is not exact science and if you ask different linguists, they usually have contradictory theories.