There is an English Customs desk. You need to get your forms stamped at the English Customs desk and then mail back the forms.
Also, my experience is that if the company you purchase the product from uses Global Refund, and you submit your stamped forms to the Global Refund Tax Counter with your credit card, they will immediately process your forms and refund the amount to your credit card in one week.
If the company you purchase from uses Premier Tax, then the Premier Tax Counter collects all your forms, throws away your envelopes and tells you they will refund you in a month. It's been a month however, but I have not seen anything yet. In future, I will probably opt to take back whatever I can in cash at the Premier Tax Counter.
At London Heathrow, the officer advised me that the Hermes purchases in Paris needs to mailed back to Paris for tax refund. I also need to buy a stamp and drop it in the Post Box right infront of the customs counter.
Unfortunately, the SMYTHE Store in London Heathrow has a very long queue and I didn't have enough time to buy the stamps, so I brought back the forms to my home country and had them sent out via registered mail. It has been 4 weeks and still nada...