Hi @Jonathan1253
https://www.airbnb.co.uk/help/article/2701/extenuating-circumstances-policy-and-the-coronavirus-covi...
I am not sure what the policy is for guests who are already half way through their stay. I would perhaps ask Airbnb through their social media.
If you aren't completely reliant on your Airbnb income to cover essential household costs, I would probably look to refund.
Also can I suggest you add your location to your listing rather than just saying greater London. i.e. East Sheen, south-west London. And in the description say where you are near i.e. East Sheen 15 minutes by train into Waterloo/central London, great local bus service, close to Richmond Park and lovely local cafes, shops and restaurants.
And yes many new hosts get caught out by the double discount. Personally I would never offer a new host discount as Airbnb give you an artificial boost in their rankings when you first join so you will get bookings anyway in most resorts and tourist cities.