Depends on what your _future_ flying pattern ist.

You can get with the challenge on American Airlines with Platinum in probably one flight but need to retain that at 50K without flights over to the US with BA.

That flight with BA will give you already 320 points of the needed 400.

Add one Biz flight to LHR for ~ 250 Euro after that and you exactly hit 400 points and will have silver for one year after completion. This means lounge access on oneworld flights.

If you reach 800 after that you can get Gold with more benefits.
You will also earn miles with BA which will allow you for the future (given the availability) upgrade WT+ via Miles to CW. You only can do that with BA miles.

Register NOW at once calling the BAEC tomorow and request if you can be enrolled in the current earnmoremiles promotion (ba.com/earnmoremiles) I assume chances are zero to very low, but if you do get it, you can have additional 50K miles.