As the UK is technically still a member of the EU, I just wanted to be sure of this before booking travel arrangements.
The idea is to arrive in the UK and stay there for 2-4 weeks, and then afterwards relocate to and stay in mainland Europe for another 80-90 days. Is this possible? Or would I be effectively limited to 70 days in the EU after staying for example 20 days in the UK?
The 90 day clock starts when you arrive in mainland Europe.
The UK isn't in the Schengen Area, so your time there has no effect on your Schengen time limit.
It's worth noting that the EU and Schengen Area, while related, are different things: several countries (UK, Ireland, Bulgaria, Romania, Cyprus, Croatia) are in the EU but not Schengen, while some (Switzerland, Norway, Iceland) are in Schengen but not the EU.
Your stay in the U.K. does not count towards your Schengen stay limits. They are completely separate visa regimes.
Even if you stay for a whole 6 months in the U.K., it still is 0 days in the Schengen area.
Your proposed plan is perfectly fine. Your 90 day Schengen limit will start when you enter the Schengen zone, not when you enter the U.K.
