If you want a blocked seat, I’d recommend sitting towards the back of the cabin. People seem to select their seat from the front to the back by preference on top of which passengers who don’t select seats seem to often get dumped further forward. It is a frequent bugbear of mine that from LCY the first five or so rows of CE will be fully occupied and then everyone at the back of CE will have two seats to themselves. Often this is taken further these days and all of CE is full whilst there are many empty seats in ET which is quite infuriating.