Do note that you might have to push a button (ring a bell) and/or pickup a dedicated phone to get attention at the counter.

Probably won't need to, as it is such a busy international airport, but certainly is the case at other airports even big ones! And at 'dead' (low volume) times.

Also, as a general tip, make sure you have an envelope with you. Sometimes customs (when you have had to contact them by the courtesy phone) ask you what you have bourght and where you are going, if satisfied, they then ask you to leave the form in their dedicated mailbox and they will rubber-stamp it later, and post it to the refund company.

Again, should not normally be a problem at LHR, they should be willing to come to the counter, but you never know.

This then means you have to accept either a cheque mailed to you by the refund company or a refund onto the credit card used to purchase, etc.
[If stamped there and then, you sometimes (depending upon refund company) have the option of a cash refund from travelex etc there an then. Although often there is a fee for the cash route.]

As I said, should not have a problem, heathrow is so international, but at any other airport I would be certain to carry an envelope (if the refund form from the shop did not come with one), even just a 'hotel' branded one from inroom stationary, just so you can write the refund company address (from the form) on it incase you need to post it into Customs dedicated letter box for them to stamp then post to the refund company (if you do need to do this, also get a local postage stamp from WH Smith etc, at the airport if you dont have one to put on the envelope - if it is not freepost)

I honestly assume you wont have any problem at all (unless you are on a very tight connection and also a huge unexpected big queue, etc).

Just mentioning it in passing as a tip for others.