I am thinking of fitting one to collect my waste water to use on the lawn
A) Are they easy to fit
B) Can I use all my water (such as washing machine and washing up water)
Usually easy enough to fit a diverter to a down spout to collect rain water.
Using 'grey' water is more problematic - unless you live on the side of a hill it isn't all that easy to divert your kitchen outflow pipes at a height that will fill a water butt. The other problem is that storing grey water in a butt for any length of time can be a bit scummy/smelly - although your lawn won't care about that :)
You ought to be able to divert all first floor grey-water to your water-butt. But you would have to find some sort of system that would allow you to 'switch back'. If you made a permanent alteration you will find yourself in an 'overflow' situation when the rains come.
Strictly speaking you need to fit a gulley under the water-butt to cope with any overflow situations.
If you can identify the down pipe and it is separate from any 'foul pipe', then why not. You may get a bit of soap scum forming, but that shouldn't be too much of a problem.
I can remember syphoning bath water onto the veg plot through a hose-pipe during previous bans - which is *not* illegal. It's only using a hose connected to the mains which is banned. How often you have a bath with cold water is of course completely up to you ....