Not that I know of, it didn't a year ago...but it could have changed, I don't think so though...
I haven't seen Miu Miu in any London airports yet. I'm flying out of Heathrow T5 this Friday. I'll let you know if they stock Miu Miu there.
As promised, the Prada boutique in T5 doesn't carry Miu Miu. The harrods store has Miu Miu sunglasses for sale. Other than that, there's no Miu Miu in T5 I'm afraid.
You can still buy Miu Miu in London and get the VAT refund at the airport when you leave.