As Karl states, there is no 'duty free' shopping within the EU, only 'duty paid'.
Eurostar security is simply a matter of having your bag X-rayed. There are no restrictions on liquids or sharp objects. All passport checks, with Eurostar, are prior to boarding. (i.e the French National Police will check your passport at St Pancras and the UK Immigration service will check your passport before you board, for your return journey, in France).
Other than that, travelling on Eurostar is just like travelling on a train in the UK.
As far as tobacco and alcohol are concerned, you're allowed to bring back as much as you like as long as it's for personal use. HMRC have a guideline figure of 3200 cigarettes as the maximum quantity that they'll normally accept as being for personal use, without asking any questions. (However, they might challenge you if you brought back lots of different brands, even if you didn't exceed 3200). There are also guideline maximum amounts for alcohol but you'd find it hard to exceed them, simply because you wouldn't be able to carry that much:
http://customs.hmrc.gov.uk/channelsPortalWebAp p/channelsPortalWebApp.portal?_nfpb=true&_page Label=pageTravel_ShowContent&id=HMCE_PROD_0102 21&propertyType=document
In practice, you'll hardly notice any HMRC staff at St Pancras. Basically, you simply step off the train and go straight to the taxi rank (or wherever else you need to go) , in the same way as you would from a domestic arrival.
Chris