EAP86 - yep the fin fuel is about what you use for start-up, taxi and take-off. In effect this gave you full internal fuel available for use (plus whatever external fuel) having achieved about 350kts+.
As for AAR the fin tank takes an age to fill and without a fuel gauge it can be a frustrating wait to achieve Fin Full. Not all users went for a wet fin either; the Germans and Italians did not include it on their IDS aircraft.
The use of fin fuel on UK aircraft has been on and off for various reasons.