I've booked a one way on the direct train and then a return via Lille at a later date, if that's what you mean. I got a great price for the outbound direct train when the tickets were released but I wanted to come back on a Tuesday so had to wait until three months out before I could book my indirect train home. I had no problems doing this. The only small issue I had was that I couldn't book the one way via Lille on the UK website, only the French website. I have a feeling this may have changed since then but I'm not 100% sure.
Bear in mind that when booking indirect, the trains to and from Lille are released further in advance that the whole journey to DLP. As a result the prices may have gone up before the TGV prices are released three months out. I booked indirect twice now and have got the cheapest tickets at the three month mark, however if you're booking for peak times or don't want to take the risk then you might want to book the Lille-London tickets when released and then the TGV bit later on using the websites mentioned by Carrie Cat above.