I suspect Instant is an internal reference, since they differentiate between customers who have attained it (organic) and those who got it on a match (instant). I agree that there is no lost benefit - well, you can't use the Gold line, but the regular one is perfectly acceptable, especially when you compare it to the abomination that is BA and their call centres.

In any case, they're doing you a favour by delaying the start of the status to the day before your flight, thus prolonging its effectiveness for you.