Rental offer accepted for a week, landlord's house is still on the market

I put in an offer for a terrace house in London about 10 days back, 8% below asking price. Last week the agent came back saying that the landlord has accepted the offer, but is in the process of obtaining gas certificate before they can accept the deposit. They kept the rental listing on as of today. This is my first time renting in the UK (relocating for work) - is this normal or are they keeping me on hold while waiting for better offers? Appreciate any insight or advice!