From 1st July, all 0800 numbers are going to be free in the UK. Some helplines (such as CALL helpline in Wales) use 0800 and some people have been reluctant to contact them because of cost.
0845 numbers (such as the Samaritans) remain charged for, though their cost is liable to change - this is dependent on your network.
For more information, see here
http://www.ukcalling.info/