Who do I call for Google Tech Support?

You have to go through the Google help section online.There is no number available publicly to call the Google Store for support as far as I am aware.

If you have a Google One membership, Google manage account and you will be able to contact them directly with the number located there. I'm not sure the staff in the store can as they only know the physical products. Not the software.

Tech support is available in store. Visit the location in Chelsea