dj Tactical voting is up to the individual and doesn't require any negotiation. It's what loads of people have been doing for years anyway.
A progressive alliance is something different. That's when a party without any realistic chance of winning agrees not to field a candidate and extra canvssers, etc are sent to a seat. There have been a couple so far. I think Vauxhall is one of them. I think the Labour Party in Jeremy Hunt's constituency is deciding whether to drop out. JH has a 24,000 majority, so it's going to be difficult, but impossible if the opposition is split. I can't remember where the other one is, but I think it involves the Greens, who will probably be relieved to save their deposit.
It's easy for me too. I'll vote for my preferred candidate, because it won't make any difference.
TBH, it's like using a bucket to bail out the Titanic. Labour are forecast to lose 100+ seats. All tactical voting and/or a progressive alliance will do is possibly reduce a Conservative landslide by a few seats. The Conservatives are going to have a blank cheque to do whatever they like, so it's a question of damage limitation. There aren't many seats where individual votes even matter.
It wouldn't be a surprise if there's a new left of centre party ready for 2022.