This seems to be a possible glitch on the Zavvi,com website, to do with the £3 off £30 voucher code, MVC3. That voucher works for £3 off £30, or 3 euros off 30 euros.
If you want an item that is just under £30, you can get a £3 discount on it. Here's how:
Log in to your account on Zavvi.
Go to item page on Zavvi.
Change currency to Euros using icon at top right.
Add item to basket.
Go to basket and apply voucher code MVC3.
Change currency back to pounds.
Because changing the currency over to euros makes the price go above the 30 euro mark, you can apply the voucher code and it sticks whenever you change the currency back again.
For example, Naughty Bear for the PS3 is £28.95. Change to euros, add to basket, apply voucher, change to pounds and it shows up at £25.95.
UPDATE - if you are seeing the error "A discount code has already been applied to your basket" - try repeating the above steps again when you are signed in. You should still see items at the lower price.