100% scholarships are incredibly rare, and in fact are not permitted across the schools which are members of the Headmasters Conference who limit their maximum scholarships to 50%. In practice, few schools offer that many 50% scholarships, preferring to offer more smaller ones - some offer scholarships with no financial reduction, just the kudos of being called a "scholar"!
If you are entitled to apply for a bursary, these can be 100%, as they are means tested - they can also be less, on a sliding scale related to income. It is possible to have a scholarship AND a bursary, so, in theory, this could amount to 100% fee remission. (Remember scholarships do not cover additional costs at the school like trips, uniform etc - bursaries might...)