Transportation from Belgravia area to St. Pancras station - Eurostar

Depending what part you are in the Piccadilly line might do it for you.

Taxis take a maximum of 5 people, so 2 would be needed. There are plenty of public transport options.

I think the Tube will be loads easier than a taxi, but I guess it depends on what time of day? Will you have luggage? Your hotel can probably assist.

We each have a smaller suitcase (carry-on) and would be going to station between 3:30 and 4:00 PM. I
suppose that is a busy time on the tube.

Not particularly busy, certainly nowhere near as busy as an hour later. You are aware that you need to check in - not in the queue to check in but actually checked in and approaching security - 45 minutes to an hour prior to departure time? Probably more at the minute since we are on the highest alert status.

Absolute drop dead check in time to scan your ticket at the turnstiles before security was 30 minutes on Thursday. But 45 to 60 is safer as the gates lock down at 30 minutes.

There was a French woman absolutely freaking out on the platform just before we pulled out. I think someone in her party missed the check in or boarding. I could not make out everything but the guards kept repeating the 30 minute deadline.

Thank you for your help!! We will take the tube, as recommended, and will make sure to be there early.
(Another thing I can now check off my list!)