The volume on Bittylicious is low and it's effectively peer to peer, not an exchange.
I don't think any retailer doing thousands of transactions a day is going to risk waiting for Bitstamp to do a BTC - USD - EUR - GBP conversion when their entire profit margin might only be 1-5% and that could vapourise into a loss instantly. Overstock and anyone else in the US goes straight into dollars.
Yup, without a real exchange, and a real payment processor, there really can't be a big player accepting bitcoins in the UK.