My best advice is to go on Spareroom.co.uk and have a look at the other rooms for rent in your area. I used to rent out a spare room in 2 different properties West London over 12 years so have lots of experience. In my situation it was sharing with just me or then me and my boyfriend/husband.
I would think carefully about your ideal lodger and how you like to live. What sort of person would fit in? Would you ideally hope they're away over Christmas and at other times? I liked to find out about their lifestyle and family / friend's situation. If they have a boy/girlfriend would you be happy for them to stay etc.
What's their working pattern. Will they work from home and what will that look like?
Rents have changed especially since Covid. The room is worth what people are willing to pay so think about your marketing.
Describe the location and transport links and put them in the ad. Is there secure space to store a bike? Parking situation (most won't have a car). Traffic/tube/flight noise (don't mention unless there is none, but will affect value)
How large is the room? Is there additional seating and/or a desk? TV? The quality of the furnishings and mattress, is there plenty if storage - this is absolutely vital for renters. Under the bed, wardrobes etc... Do you have a cleaning service, will they bring their own bedding, will the cleaner change the bed each week and wash it or will they do their own... which shared areas of the house and garden are open to them...
Finally get some great photos of the room and a few of the wider house.
It all adds value in a competitive market in Zone 2.
I always invited a prospective lodger for drinks or supper to see how we got along. Will you and the family feel relaxed with them in the house? Do they have a great social life and what do they do at weekends and in holidays? My first ever lodger married my friend and my second became my bridesmaid 10 years later but others had their own lives and we got on well when we saw each other (not a lot! That's the great thing about London) but didn't keep in touch.