I'm sure there are some items that are available in London that aren't here. But I'd be hard pressed to name any.
Unless your sister just happens to have a couple of hours to kill in central London - I'd forget about it.
Don't get me wrong. I was just in London and spent a very enjoyable couple of hours wandering down Jermyn and Old Bond Streets, fingering the mechandise at Trumpers, etc. But that's the whole point - and about the only advantage - of buying shaving stuff in London: You get to physically put your eyes and hands on some of the high-end razors, brushes, creams, etc.
But you certainly won't get very good prices. And virtually anything you might be able to get at any of the high-end London stores is actually available through the web or mailorder here in the US.
I suppose Boots does offer a couple of shaving related items that aren't available here. But they are going to be available at every Boots outlet (comparing US drugstores with British ones is sort of like comparing a Little League field with Yankee Stadium).
Palmolive sticks & cream
Erasmic sticks, soap & cream
Boots sticks, soap & cream
Kent soap (if she's near a John Lewis)
Wilkinson sticks (I find the soap near useless - but hey it's like £1.50 or something and you might like it)
The second link in my signature will send you the right way for the type of stores for her to frequent on your behalf.
John Lewis for Men-U, Kent and Tweezerman brushes, Kent, L'Occitane C&E and Burt's Bees soap/creams/scents. Boots for Israelis, Real Shaving Compnay Shaving Cream and other things. Also, if you have a delvery adrss, it might make sense to order online things made in the UK and get them sent to a UK house. No hanging around and you save money on postage