I assume that the arrangements at St Pancras are the same as they were at Waterloo (where I've arrived on Eurostar on many occasions).
If so, you pass through security checks (bags X-rayed etc), French (or Belgian) passport control and UK passport control in Paris, Lille or Brussels. There are no customs formalities at either end. All you'll see is a few customs officers (and immigration officials) watching people as they arrive at the London terminus. I've occasionally seen immigration staff ask non-white people to step to one side for a few questions but I've never seen customs staff stop anyone. (My reference to 'non-white people' shouldn't be construed as racism. It's a simple fact that I've only ever seen people of Asian or African descent stopped by immigration staff)
I'm sure that the procedures at St Pancras will be just as quick as they were at Waterloo, where I would normally expect to be standing at a bus stop, or on a Tube platform, in less than 5 minutes from the train arriving at the platform.
Chris