I am traveling from Strasbourg to London and have found that going via Lille is both cheaper and easier than changing train stations in Paris. The Eurostar leaves 39 min after the TGV arrives from Srasbourg. Is this cutting it too fine? How long is the walk between the two stations? What happens if you are a bit later than the 30 min check in time for the Eeurostar? Any other tips?
I'm not sure about Strasbourg, but there is a 30-minute minimum check-in time for the Eurostar trains in London and Paris.
Walking from Gare de l'Est to Gare du Nord in Paris takes less than ten minutes. Walking from Lille Flandres to Lille Europe will take a bit longer. Since there is a minimum 30-minute check-in deadline for the Eurostar, your plan won't work. Where are you getting your fares? The lowest discount fare for the Eurostar from Paris to London is the same as from Lille to London (€50). The cheapest Prem's fare for Strasbourg-Paris is €25. The cheapest fare for Strasbourg-Lille is higher. What is your travel date?
These plans are for the end of September, so unfortunately we have missed the cheaper fares. I am checking the Eurostar and TGV sites for the cheapest fares and working with them. At the moment the Strasbourg-Lille and Strasbourg-Paris fares are the same, but the Eurostar is cheaper from Lille.