I'd be interested, but like others would need to know a lot more.
On timing - Britcon is in August, and there are further competitions each month up to November. Many people will be cautious about giving up two weekends in a month to wargaming no matter how fanatical they are
Also, a one day event is preferable to keep the travelling costs down to an acceptable level, but that does result in a long day - but if Tiny-tim brings his missus shopping, he is facing a very early start and late finish if you adopt the usual UK timings. (I think we agree you need three games to get a reasonable result (possibly more if you want to get 'newbies' interested), each of around 2.5 - 3.0 hours long. With breaks that is 8.5 hours minimum.)
On a more practical note, have you / the GW manager considered the practicalities of space in the shop, which is likely to be limited if you also have GW customers involved as well as 16 burly competitors? It will also mean that it is likely to turn into more of a demo than a true competition
On format, if you want it to be truely "blind", then I would agree that Siegers could be troublesome - best to limit the format to the known three (Marines, IG and Orks). However, 1500 points will automatically restrict titans to the smallest ones. I am less concerned about having two SCs though, and if both people bring spaceships, then they are played as a single activation with a single ship (ie one person loses an activation). If using the more usual paired entries approach, then the Space and Titan restriction could possibly be applied to the 3000 point combined total - to the total value of both armies must be 1000 points or less - though it could all be provided by a single player.
Either way, I would suggest that we use the usual alternate turn mechanisms, the players on a side deciding which formation is to be activated.
Of course my Warlord titan is unstoppable 
