Will it work? This sounds promising, but it is hard to stop cyber rip- offs. Anyone can copy images, including "e-hallmarks", from a legitimate site, and if the DTI does catch up with them, that might be too late for those who gave credit card details.
Many FTSE-100 companies would fail on this code of practice, including having a phone number. A more useful idea would be to educate the public about what to expect from a well-run site (such as a "secure" electronic server for transactions, which costs more than crooks can generally afford) and spotting fly-by-night outfits by their web address, which will be "hanging off" a small-time or even free web space provider, rather than having their own paid-for "web domain".
A knowledgeable public is a far better foil for fraudsters than easily faked "e-hallmarks".