Is this phone still available from 02? It shows up in a direct search on the store but doesn't appear in the device list if you filter by manufacturer.
I'm using a L530 at the minute as my L920 suffered a nasty fall, I'm two months off an upgrade and like the specs of the 1520 but with it not showing up in the store consistently and it appearing to be out of stock when I do find it I'm worried that it may have been discontinued.
Yes It's showing out of stock right now via -> https://www.o2.co.uk/shop/phones/nokia/lumia-1520/ so maybe if it's convenient contact your local O2 HighStreet. Store to see if they have one in stock. It does look like its being superseded so may be limited in stock.
Ordering online may leave you with a wait with it being out of stock, and because we are near to Christmas.
If your buying outright or on payg then perhaps look at Carphone Warehouse. Their handsets will be unlocked too. (Ask in store to conform)
Or if on contract again Carphone warehouse as long as not having the refresh tariff is not important and they only offer standard 24 month contracts on O2