I am travelling to Portugal and Paris in December. I am from South Africa and will be flying on a Schengen Visa. We want to do a day trip on the Eurostar from Paris to London. What are the visa requirements I would need?
As far as I know, the requirements are exactly the same as if you would want to travel to the UK for a month as a tourist. There are some rules about transit but no exemptions for day trips or for Schengen visa holders. And trains to the UK are treated like flights into the country, which means passport check and security inspection prior to boarding.
In your case, it means that you also need a visa for the UK.