so it's under 14 days before flight EU261 kick in...maybe You can just buy LH or Lot ticket and claim money from BA, check forum for EU261.....
also You can be rebooked on WAW-LHR on BA 851 on 19 Aug.....its I think 4h by train...but BA will not cover train ticket
If you have insurance on payment card can be more options...I always pay with Amex Platinum