I will preface this by saying that I’m only responding based on my own personal experience on LOT. I don’t know the technical ins and outs of how seating works.

If you’re trying to grab a seat without a neighbor my advice is just to ignore the seat map until just before boarding starts. Luckily you can go back through mobile check in and change your seat up until the last minute.

My experience is that blocking is completely random. Even as a HON expertflyer has shown the seat next to me blocked, only for it to be filled during check in (I’m talking even on a GDN-WAW flight that was maybe 60% full).

I’d expect the LHR flight to be pretty full though so there’s not much that you can do