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.
Credit:stackoverflow.com‘