It's tight, but doable if your flight is on time and immigration isn't too bad.
One word of warning - the current date for Brexit is 29th March. If it's on or after that date, it's very difficult to predict what will happen. Worst case scenario, you may see flight cancellations and severe immigration delays. But it's still very much up in the air.
If things are normal, then it all depends on what the non-EU immigration lines are like at CDG T2A, which BA uses for arrivals. You are signposted to a shuttle bus for the TGV station but, in reality, it's a pretty easy walk - go out past T2C and into the station. There's no need to check in for the train, you just get on board. Hopefully you've got e-tickets!
Note that there are two major stations at Lille: Lille Flandres and Lille Europe. Your train will arrive at Lille Flandres (at 19:46). There are onward, local trains to Belgium from here. At xx:08, the Antwerpen/Anvers train takes you to Kortrijk/Courtrai, from where there is a cross-platform connection (the trains pull in simultaneously) to Brugge/Bruges (dest: Oostende/Ostende).
I suppose one other thing to consider is food. I don't know if you'll eat during your transfer at LHR, or get something to take on board, but you may not have time to pick anything up at CDG. TGVs do have a buffet, but the offering is fairly snack-like:
http://medias.sncf.com/sncfcom/pdf/r...te_Bar_TGV.pdf.
Depending on timing and your level of tiredness, you will be arriving at Lille at peak dining time. The station is right in the city centre, and Lillois food tends to be rather good (and the Grand'Place is rather an impressive place to see).
Good luck!