From my housekeeping records I know what I spend in Asda each month, so I will probably buy a year's worth of cards (£280 on each) to maximise the bonus. I'll treat them as "near cash" in terms of my cash safety buffer, so it's a nice way to get a boost on rainy-day money.
You can register the card(s) online at asda.com/giftcards which allows you to declare it lost/stolen and transfer the balance to a replacement if necessary. You can add cash on-line or in-store (I prefer to do it on-line as the cashiers can get a bit

Last year I bought about four month's worth and used them without any problems.
VRD
(no connection with Asda except as a customer)