I am in London now. UK VAT ended with Brexit, so no, there is zero benefit if you are going to London to save.
All you save is US taxes. However, you may now need to pay customs taxes, so no, I would not go to London with the thought of saving money. You can check actual UK prices by going on to the UK website, and you can convert from there.
I would say this is true for high-end luxury goods. However, some items you do save. I am a fan of Frederic Malle's perfumes (Carnal Flower for me!
), and there is some decent saving there. I didn't buy any though as it's still not worth it to me to lug small stuff like perfume back home to save a few pennies. Plus, you don't get the freebie samples which can add up to a few hundred dollars, so it's a wash.
That said, I would still definitely go to London to shop. There is a lot of stock here (Chanel is packed with stuff).
There is no VCA at LHR airport, but there are other luxury brands (e.g. Cartier and Tiffany jewelry are in T3). Last time I was there a few months ago in April, the Chanel prices at LHR were about 10-15% less than the Chanel boutique. Not sure why or if they have since adjusted their price. Even so, 10-15% saving is not worth the headache to purchase a jewelry item, not for me. Bags, shoes, clothes... those are okay, but for me, I always buy jewelry in the US as the after care and building relationships is equally important to me.
Bummer though, you will just miss the special Tiffany exhibition. I flew here to London to see it; it was great. Unfortunately, it ends this Friday, Aug 19. If you are a Tiffany fan, maybe some high jewelry pieces will be exhibited at the boutique (not sure if you are a Tiffany gemstone fan).
Have fun in London! I actually now enjoy shopping in London more than NYC. Peaceful, simple, and all the boutiques are in one area. Not to mention... afternoon tea! If you like tea, afternoon tea is a must. Enjoy your trip!