grand nation… betting online?

anyone no of any sites that have good online bonus’ for first time registration..
ive had a look n they all say like £20 free bet but when you read into it they want you to put £20 on on one horse in one go? i want to deposit £20 n actually have £40 to play with or is this impossible? i no with bingo sites n stuff u deposit £20 u get £40 is it possible with any betting sites?
much appreciated if any1 can help