Natwest offering £100 to play with guys (and gals 😉)
Use these bonuses to practice. I cannot see the point of doing anything on demo when a company will "back you" so that you can trade real money, get the psychological element down (which demo doesn't help you with) and not have to worry about taking any risk with your own capital...
If you open enough you can get several thousand pounds for free.
Do a Google search for others but here is a few:
Natwest (above)
iii
E*trade
The cashback against losses offers are just as good I think. Try
Worldspreads
Shorts and Longs
Don't worry if they ask you to put money in. Just do that so you get the opening bonus. If you lose it, you can always take your own money out!