I don't think I've ever seen an item on AR which was valued at millions. I remember a water colour picture they valued which was later sold to a museum for £100,000.
they did a Where are they now? thing in the BBC antiques mag a few years ago and yes there were folk who had sold and realised the valuation and more. Do keep in mind that where the valuation is for insurance purposes, this means that the insurance company would have to replace the item at the time and probably through a dealer so it would cost more than waiting for an auction and buying at auction.
Not in private hands - there have been a few around £150k - In the US there have been a couple of big ones, a Navajo black and white rug that was on the back of someone's sofa and a major Hudson Valley oil (can't remember the artist but it fetched $950k at auction in Pennsylvania, eight phone lines open....).
In public hands, the miniature of the Angel of the North was valued at 1 mill - more recently the Penryn town silver cup was put at £150 - 200k
If you have noticed woofgang, they have stopped valuing "for insurance" - I suspect this is for that very reason of a liability rising between the BBC stated value and what the insurance co thinks, despite them gouging the premiums off the customer.