The Valiant had a 100% giveaway capability, but I think this was a legacy feature and not a deliberate design.
Not quite.
Max fuel of the BK1 was 80,165 lb with under wing and bomb bay tanks. The Pilots Notes give:
'The tanks used for transferable fuel have a total capacity of 46,248 lb, but 1,160 lb is not transferable. This gives a total of 45,088 lb of fuel which can be transferred from the Tanker to the Receiver.'