In your situation @Caroline1258 I would message back the guest and ask them to:-
a. change their profile image to one of themselves
b. ask them about plans for their stay and why they chose your place
c. ask them why they are booking a five bedroom house for one person.
If they don't respond or don't give a satisfactory response, then contact Airbnb and ask them to cancel the booking as you feel uncomfortable.
Not sure about co-host issue as I don't use one.
I would always recommend new hosts remove instant book until they become more familiar with how Airbnb works and so you can vet guests individually before accepting bookings to ensure a good fit.
By the way your price is ridiculously cheap. You are letting one person occupy a whole five bedroom house for £59 a night. Please look at pricing for similar properties in your area and adjust your prices upwards. Even for 10 people it's only £134.