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.
They will also use a similar payment processor like Bitpay or something else.
But Bitpay is dependent on a USD exchange. We need a GBP one before any large scale adoption can take place.