Hi all, apologies if this is in the wrong place or questions have already been answered before, bit overloaded with information coming here for the first time

I'm looking to fly alone from LHR to HND direct on BA5/BA7 and back on BA6/BA8 May next year, I've got plenty of Avios and a cabin upgrade voucher to use. I'd like to fly business.

From what I've read, there'll be 4 reward seats in business released 355 days before. Should I expect to have to phone up at midnight when they're released and book outbound separately from inbound? Is this route and time expected to be so in demand?

Also, is a cabin upgrade taken from the reward seats allocation? E.g. do I need to pick a premium economy seat (of which there are only 2 released) then the voucher upgrades it to a business seat (which doesn't need to be listed as a reward seat?), or does there need to be a business seat available to book and the voucher just reduces the cost of it?

Also, I see next to no availability for these direct flights on on BA's (absolutely terrible) reward flight finder page for this May, but I do see them for the next month or 2. Do they release more seats over time to fill up the plane?

I'm pretty flexible to an extent exactly when I go and come back, but want to have a plan a couple of weeks ahead at least so I can arrange things when I'm there.

So what is the best thing for me to do? Any suggestions or tips

Thanks in advance