Update:
Just seen that if you ask www.voyages-sncf.com for Paris Nord to Londres, and put 'Lille' in the via box, it will offer you the above-mentioned Paris - Lille - Brussels routing all as one ticket (for as little as €130). As it's one ticket, if the TGV was late, you'd be covered for the connection in Lille.

The journey time (on 17:46 departure from Paris) is as little as 3h11 (compared with 2h45 for the direct service - 2h15 journey time plus 30mins check-in in Paris; if you're leaving Paris on the TGV, you only need to check-in when you get to Lille).

For me, this (or even the slightly longer journey times) would still be preferable to flying...