I can recommend Sandown 6-7 March see emigrate2.co.uk I have been to this expo at both Sandown and Edinburgh
as I have posted in a couple of threads we have found these immensely helpful, both the seminars and meeting people on the tradestands. People give advice, information, booklets and offers not available elsewhere. Some of the exhibitors will also be australian so they can answer your most off the wall queries from chocolate availability to school experiences.
if you are going, plan ahead, write down what you want to find out, who you want to see and take notes of the information you are given. it can be gruelling with lots of queuing so wear comfy shoes and take a pack up!
DONT TAKE CHILDREN unless you have absolutely no option
there is also down under live on 30 Jan in Islington. (downunderlive.co.uk) I have no experience of this one but I am undr the impression that this is probably more useful for back packer trips and temporary visas. but I could be wrong. I thought about going last year but it was cancelled at the last minute due to lack of support - dont know if that indicates much
If you were ever thinking of going to an expo, Sandown is the one to go to....it's huge.
Also, as ELH suggested, if you have young ones it's best not to drag them along as it wouldn't be much fun for them and more importantly you can concentrate on which stands you want to see, ask questions etc without having to worry about the munchkins.