Can a UK research council-funded PhD change direction to fall outside the scope of the RC?
It is a serious problem if the unforeseen issues make it impossible to fulfill the terms of the grant.  You will need to work with your programme directors about that.  

If the unforeseen issues and distractions merely make you prefer not to fulfill the terms of the grant, than too you could ask your programme directors.  But don't be surprised if they prefer that you do fulfill them.