Are there in-expensive transports on 12/8 that i can get to Lille France and catch the Eurostar from there...?
Dunno. But a National Strike is just that. Nothing moves. Not trains. Not buses. Not pony and traps. Nothing.
The most sensible thing would perhaps be to either rearrange to go on Sunday evening (Eurostar says they will reschedule) or consider flying - although I bet that many others will have similar thoughts of flying and cheap may be out the window.
Another option is to travel on Tuesday, 12/9. I have a flight leaving **LHR ~ 2pm. Is Eurostar pretty punctual, so could I catch a train from Zuid station , Tuesday am, and get the St Pancras, around 10am and then catch the Piccadilly to venture onward to T5..
That strikes me as quite brave. Your tolerance for risk must be higher than mine.
In order for that plan to work you would have to be on one of two first Eurostars after the strike, either the 7:56 or 8:52, you would have to have found an available seat, the train would have to leave on time, and not run into any residual problems as it traveled through Belgium and into France. And the train would have to have a clear run through the tunnel and through England. And there would have to be no problems on the Piccadilly Line.
That would all take quite a bit of courage. More than I would have on a normal day, and way more than I would have the early morning right after a nationwide strike.
Eurostars tend to be reasonably on time but things happen. I've been on time many times but also been held up for up to 3 hours getting through the tunnel when there are issues or congestion. Belgian traincrew and trains may be out of position after the strike and there may be congestion on the tracks. It is also possible that in the aftermath of the strike not all staff will move in their customary efficiency. What would you do if you missed your flight?
I know from personal experience that traincrew aren't always the happiest in and around strikes.
Luckily you have several days to plan an alternative plan - this isn't sprung on you at the last moment.
Can you rearrange your Belgium component? Can you fly? Will the flights be flying? (I don't know). Can you get out before the strike hits - my strong suggestion.
Is this a pretty safe path as well...?
I'm sorry, I don't know what you mean? You do have to go through security to board the Eurostar. You don't have any security checks on or off the London Underground.
Good luck. Please come back here and tell us what you did and how it works out.