Hmm, I think you need some serious advice. If they are just making you redundant and no one else, they'd better have a pretty good reason why, otherwise you could quite likely haul them over the coals under sex discrimination law. Having said that, from the experiences of various friends of mine, it seems that many city law firms behave like the biggest kind of shits, simply because they know they can rely on people not rocking the boat.
Yes, I think you need to ascertain all of the things you have suggested - but I also think that you should try and find out what these are BEFORE the meeting - otherwise they have all the power. If you can't do that, then just take them on board at the meeting and don't agree to anything. But also have a very good read of your contract - I would be surprised if you were on less than a few months notice, so they would still be obliged to pay you the September payment if you were working out your notice.
But do you know anyone in employment law? Or someone who does?
And, btw, I think you probably can spin the redundancy whatever happens. You wanted to work more in your area of specialism, that just wasn't happening in your current firm and that work wasn't around. So now you are looking.