The Eurostar staff are usually very friendly so even if your EMT train is severely delayed, they should still let you check-in 10/15 minutes before your departure and would almost certainly try and accommodate you onto the next service if you missed it, even if you didn't have a through/London Int ticket.
Also, are you sure that it is the only connection? I have found this connection later (which could be used as a back-up) but it depends on what day of the week you are travelling.
This is a possible connection for tomorrow (3 May), but there may well be different times/different trains on the day you want to travel. As long as it isn't a Saturday then there may be another option.
London St Pancras Int 10:25 Paris Nord 13:47 EST 9018
Paris Est 15:24 Karlsruhe Hbf 18:25 TGV 9575
Karlsruhe Hbf 18:51 Mannheim Hbf 19:14 ICE 376
Mannheim Hbf 19:31 Berlin Hbf (tief) 0:22 ICE 592