On the few occassions that I have had to purchase online and pay large amounts, sometimes sums totalling £2000-3000 in a space of two or three days, Halifax have always phoned me to check whether the transactions were genuine. Only after confirming it, did they authorise the transaction.
Smaller transactions were also used to buy stuff from Ebay and payment made using Paypal. on a few occassions Halifax phoned me and confirmed the transactions with me.
Always pay by credit card for online purchases as i believe you are much better protected. Maybe someone can care to expand on this matter..