Well, you can also look at it the other way round. The dates might explain a lot on the contrary.
Stricto sensu, Bell Beakers were just pots - initially produced in Portugal (from 2900 BCE onwards). Then the pottery style expanded northeastwards, perhaps with some degree of demic movement. Simultaneously, or shortly afterwards, groups from the steppe people who had just arrived in - say - Bohemia, launched long-ranging incursions into western Europe. The two streams of people (and genes) apparently interpenetrated each other in a fairly complex way.
The 'steppe' foragers may have retained contacts with the groups they had left behind (you gotta visit the fam'ly once in a while!). Trade seems to have expanded massively at the time. It probably just ran along the trails those invaders had left on their way. The Beaker pots went east. R1b , its language - such as it was, some form of Proto-Italo-Celtic - and its burial practices, went west. The newcomers married local wives. Most of the time, pottery was a female occupation. And beaker pots proved convenient for drinking beer and mead. No reason to discard them. So much more reason for "exporting" them. What I imagine is a newly established netwok of trading paths crisscrossing western Europe, running both ways.
2500 BCE fits in nicely to explain the beginning of that influx of beaker pottery into Germany. From there, shortly afterwards, it was taken to the British Isles by a massive wave of people from somewhere in, broadly, north-west Germany.
The genetic impact in Iberia, though strong in terms of y-dna in places (the west) may have been limited autosomally for some time.
