Dialling freephone 0800 and 0808 from any landline should be free.  It's only when dialling them from a mobile will they charge you.

So you should be ok from SkyTalk's landline service.

Even on the odd chance you're not then as they don't do linerental yet (they apparantly are in the near future) then you can simply dial 1280 before your freephone number and the call will be carried by BT instead of SkyTalk.

Don't worry you should be ok.