Yes, EU/UK to US normally requires the transfer hall security check in IST before continuing to airside. All carried liquids must be 100 mL or less.

Even other destinations can go directly to airside if there is an agent supervising the transfer corridor after the jet bridge. If this one is missing, all will have to go through security check (happened to me on various occasions).

If I were you, I would not buy liquids over 100 mL at Heathrow airside. It will in any case be a question of luck.