At all.
Firstly lets establish the rules using the ICO's (the UK's Data Authority) handy FAQ [1], a site providing a copy of the GDPR [2], the EU Commissions adequacy decisions page [3], the EU's privacy shield page [4], and the dedicated site for the privacy shield's Google LLC page [5]:
In short (yes, this is my idea of short):
* We, the UK, are currently in the transition period where EU rules still apply [1 - "What happens now that the UK has a withdrawal agreement?"]. This lasts until Jan 1st 2021 [6]
* We, the UK, are currently on track to have pretty much the same rules (cynicism allowed) [1 - "Will the GDPR still apply when we leave the EU"]
* So Brexit shouldn't change the legality (more cynicism allowed)
So are Google breaking the law?
* The GDPR has a provision allowing third countries and international organisations to process data presuming. It is Article 45 [2]
* The USA has been approved for this [3]. Years ago [4].
* Google LLC is approved under Privacy Shield [5]
So, there is no indication of anything illegal going on. Yes they are segmenting the data incorrectly and messaging an italian as though they are a brit. However the change would be perfectly legal was it to be done to the whole of the EU.
I don't like it, but it is legal.
[1] https://ico.org.uk/for-organisations/data-protection-and-bre...
[2] https://gdpr.eu/article-45-adequacy-decision-personal-data-t...
[3] https://ec.europa.eu/info/law/law-topic/data-protection/inte...
[4] https://ec.europa.eu/info/law/law-topic/data-protection/inte...
[5] https://www.privacyshield.gov/participant?id=a2zt000000001L5...