Excuse me, do I need to exchange my boarding pass before going to customs for stamping at Heathrow Airport ?

Yes, check in first, do not check in luggage at all, it may be inspected, and then check in after refunding.

Each terminal has tax refund counters both inside and outside security. If you choose the counter outside security, you will first process the refund. The refund form can be stamped and collected directly at the counter, and some mall refund forms that have already been refunded must also be submitted. Generally, they will not check your refund items, but if they are very high-value items such as watches and bags, they may check them. If you want to collect money before security, you can only choose US dollars, Renminbi, Euros, etc. If you want to collect pounds, you must go to the counter after security, which means you will have to queue up again.

First refund, do not check luggage, because you may be asked to view the things you buy. We checked in first, and then asked others for a long time. Fortunately, the girl who handled it was kind...

I took Qatar Airways, and his ground staff asked me to go to the tax refund first, and then go to him to check in. The staff at the tax refund counter showed me the ticket information and helped me handle it. Moreover, on the day I left the country, about 4 hours in advance, there was no need to queue at the tax refund counter at T4, and there were very few people.