First off, congratulations! I am in a similar position myself, but perhaps a few months ahead in our planning. We'll be cashing in avios for First DXB-SEA when we get the availability!

I can't help with much of the below, except of course you won't be able to look at Aug-Oct 2014 for a few more months.

Also to use the 241, I think it has to be a pure redemption booking, ie not UUA.

But that's not bad news in your case, for your 100k avios and 241 you will likely have to pay only a couple of hundred £ extra (+taxes etc) to scoop your 2x First returns.