I recently travelled Derby-Lille on through tickets bought through the Eurostar website. The DBY-STP component were advances, but at a price lower than anything EMT offers ... £14 down and £18 back IIRC. They gave us an hour to connect going (which we needed because we were 20' into London due to the weather) and about 45' between trains on the return.
There were extra passport checks on arrival back into London due to the infamous 'Lille Loophole', but the train was not especially full so we got through quickly. A full train and it would have been tighter.
Interestingly, when I travelled London to Clermont-Ferrand in October (also through booked from E*) I don't think the Paris-Clermont ticket was any cheaper than an SNCF Prems ticket. But I'm glad I booked that way as there was a power failure on the RER crossing Paris on the return leg, and I missed my Eurostar and had to be rebooked.