I see the logic on duty, but I'm fairly sure HMRC won't . . .
With citrus, why not have a go with something like Kumquat? You're right about freshness and of course that does also make a difference to the quality.
If you don't want to go to the trouble of distilling, most citrus oils (as well as quite a lot of other things) tincture very well too - but I'm drifting way off topic now, so I'll stop there!