Travelling from London to Paris by Eurostar, you'll go through checkin, then security, then (as of 2015) a Eurostar-run UK exit check, then finally French Immigration. You won't see anyone from the UKBA, but you will see someone from Eurostar who'll scan your passport and send UKBA the details electronically, but won't stamp it. The only people properly checking your passport + stamping it if required will be French.
Travelling from Paris (or Lille) to London by Eurostar, you go through checkin, then French exit Immigration checks, then UK entry Immigration, then security.
The UK does not have formal exit immigration controls, but the UKBA do receive details of who goes from most transport providers. In the case of Eurostar, that's done by a member of Eurostar staff scanning your passport with a nifty handheld reader between security and French immigration.
France, following Schengen rules, does have formal exit immigration, with checks done by immigration staff.
So, UK -> France by Eurostar your checked "entering" France in London, France -> UK you're checked "leaving" France then immediately "entering" the UK (all in the station). All "entry" checks are juxtaposed border controls.