Is Coincraft In London A Good Dealer?

They are well known and I doubt very much they would sell a fake or replica without clearly identifying it as such.
However, I would say they have a reputation for being very expensive for what you get. Also, if you are buying a generic coin without a specific photo, the grade may not be as high as the example coin depicted in their adverts.

with the above.

They are expensive and they sell quite a lot of common coins like Churchill crowns for far more than their real value. However, for scarcer coins and ancient / hammered coins they often have some very nice items in stock and my experience is that their grading is always spot on. I've bought several hammered coins from them, and a few modern World coin sets, plus a few banknotes from the rummage bin in their shop opposite the British Museum.

I recently bought a 2006 1/10 ounce silver Britannia from them without any issues at all.