BR
Party agents are permitted to be in the room when the postal votes are opened, just like they're allowed to observe at the counts.
(Also, today I was doing a bit of phone canvassing for a candidate, and one person told me that they'd already voted by post, and who they'd voted for. I think that I wouldn't be allowed to ask them, but if they volunteer that information, that's fine.)
I wouldn't have thought that any vote could be opened before the polls close on Thursday.
And as for the postal vote point - the system is at fault there as there is no way the candidates should even be able to get into a position where they know any details about postal votes which have come in.
(Also, today I was doing a bit of phone canvassing for a candidate, and one person told me that they'd already voted by post, and who they'd voted for. I think that I wouldn't be allowed to ask them, but if they volunteer that information, that's fine.)
I wouldn't have thought that any vote could be opened before the polls close on Thursday.