" Any idea his I can claim VAT refund from London Heathrow if I order online with delivery in London ? "
Yes, provided you take custody of the Rimowa case in person, and depart from the EU with it as your personal or checked luggage.
2 suggestions:
1, Have the seller/store make duplicate receipts and provide you the VAT rebate form when you pick-up your Rimowa case.
2. Plan on extra time to process your VAT rebate at departure, just in case.