Use your MR points to travel on Eurostar will cost you 15k points.
Even if you transfer points to Eurostar miles at ratio of 1500 = 100. That still require 900 miles (13500 MR points) for a return ticket from London to Paris.

But, If you transfer your MP points to BA avios then combine your BA avios to Avios (avios.com, create a new account under same name and home address as your BA executive Club) then you only need 9000 avios(equals 9000 MR points) for a return ticket. You can also use part cash part avios as well.
Much better deal than directly from Amex.