I went to the London show back in February and found it very useful.
I second the advice about doing your research beforehand, however! I turned up with a spreadsheet of all the products I was interested in, and the best price I'd been able to find them at. I think without this information I'd have been a bit lost about what was a good offer or not.
At the show I had the opportunity to 'play' with the various products I'd researched, and managed to get a few things quite a bit cheaper or with additional things thrown in for the same cost. i.e. I got the pram set around £100 cheaper with some blankets and such thrown in, and I got the cot £40 cheaper with a mattress thrown in. I also picked up lots of little things like sheets relatively cheaply.
I'd recommend turning up early, we were there for opening time and managed to get the most done in the first hour. After that, it started to get a bit manic busy and trying to get the attention of a sales assistant was a bit of a nightmare.
If there's a Mothercare store there, don't bother too much with it. They offer a 10% discount at the show, but the staff were clueless and they had virtually none of their stock. You can normally get an equivalent discount on their online site if you need anything!
With the Philips store, get there early if you want to get anything from them. They sold out of all their decent offers very early on, and the staff didn't have the appropriate clearance to move some of their offers to other products. (i.e. I already had a steriliser from elsewhere and would have quite happily swapped that product in the offer to a newborn set (cheaper!) as it still worked out good for me, but they were unable/unwilling to process that.)
The smaller companies and independent stores were much more helpful and willing to reach a deal, as well as bend over backwards to provide information and any assistance needed.
I didn't really stop to watch any of the presentations, so can't provide any advice on those, sorry! I was there to shop shop shop.
Hope that helps a little!