Originally Posted by
efamous
You'll have to get the Customs stamps done at LHR because that's your point of departure from the European Union. Theoretically, this means that you can't check in at MAN anything that you're claiming the VAT for because the Customs officer would be within their rights to ask to see the item(s) at LHR. They don't usually ask to see the item, but it's hit or miss. The only ways around this would be 1) bring the VAT refund items in your hand luggage or 2) only check your bags through to LHR, claim them, show the items and get the Customs stamps at T5, then re-check your bags on to JFK.
I have no experience connecting within the UK, but can say that at other EU airports, I've been able to get my forms stamped when subsequently connecting at another EU airport upon showing my BPs or itinerary. (The only exception was at one of the Milan airports, though I don't remember which; the other, which I subsequently used, was fine.) From a rather long ago thread on FT, I believe there may be some rules on the length of connection time (though I'm not sure whether they are country-specific or apply to the entire EU); this makes a lot of sense, because even with a comfortable layover, if your first flight is late, and/or you have a terminal change, you may end up without time to see the VATman. In any event, why don't you give it a try at MAN and if they won't let you do it, then do it at LHR?