I’m with Talking Cars on this. Save your cash and don’t buy an additive. Just fill up with top quality high octane. In my experience, Shell is the best by far.
If you've quite a lot of fuel in the tank already, then it would make sense to bang in some high octane and Reddex, so it gets mixed in the tank. Then top up on the trip up, as necessary. However, if you’re planning to have her off the road for a while, try to leave her with a fairly empty tank. That way you can add fresh high octane when she comes up from her slumbers, counteracting stale fuel starting problems.
Enjoy the drive. When Roxy’s behaving I do Ayrshire/Skipton/Leeds on a Monday; business mileage during the week and then Yorkshire up to Ayrshire on the Friday. That can mean anything from 500 up to approaching 900 miles in a week - and not a boring one amongst them.
Safe journey.
Winko