ok, between the deathadder, sensei, m500, g403, you got 4 completely different mice. i'm trying to not be rude but there is a world of things to know about mice and i'm not getting the feeling you know much of that stuff.
so, WHAT DO YOU NEED YOUR MOUSE FOR??
Laser is generally considered an inferior alternative to the antecedent Optical sensor, and this was fact until a few years ago, when many laser sensors caught up with opticals. Still, laser are generally very temperamental, and have no discernible advantage over the opticals.
I assume that you like the intellimouse because of the very comfortable shape. The DA is way bigger and heavier, and shaped in a weird way. The M500 and G403 are very tall and require a very different grip from the DA, or the IM, or the sensei.
The only modern, high quality mouse which resembles the IM is the FK1, however there are a TON of Intellimouse available online, they simply aren't called the Laser 6000. If you google "intellimouse" and look at the images, you will see how many different models exist, and they are all VERY similar to each other. I've used WMOs, IMs, IME3, they are all slightly different implementations of the same mouse. The feel is very similar between them.
I can see right now a Intellimouse 1.1a for sale at 27 Gbp
http://www.tabletphonecase.com/microsoft-intellimouse-optical-1-1a-gaming-mouse-white?___store=uk
and you can find a ton of these as new-old stock. You just need to look.
Otherwise, we can't really give any real advice on what mouse to get. Depending on what you need, we can narrow it down a bit, but you got A LOT of choice... most of which will be quite a lot expensive.