I live in Bristol so can help.
Renting a home on Airbnb is not like renting out a house share, there are higher expectations around the quality of the accommodation. I wouldn't recommend renting as a four bed but as 2 doubles and a single with a high quality sofa-bed for larger groups.
There has been a huge surge in shared and whole lettings in the city so this gives guests much more choice and less bookings to go around.
12% sounds incredibly low for a a Airbnb host management company to manage a letting, particularly if they only charge if they let out a property. Surely there must be cleaning fees in addition?
What do they include in this fee?
For example do they have approved suppliers and will organise for a shower or boiler to be fixed? Do they meet and greet guests and do a check out?
I would advise against letting individual rooms if you aren't directly managing the property. If you do who will be responsible for keeping communal areas clean, You would also have to have cleaners go in when you have guests staying as they will be staying over different time periods. Other hosts have reported problems with noisy or messy guests upsetting other guests in shared accommodation.
Which part of Bristol will you be in. In most parts you would struggle to get 30-40 a night if you are expecting people to share with others, rather than with a host.
Before you meet this company do some market research. Look at what others are charging for similar properties in your part of Bristol. How much of the time are they full (check their calendar as an indication - although some may just be them blocking it out rather than actual bookings). What are they charging? What discounts do they offer.
Put yourself a spreadsheet together with accommodation details, what they charge, their availability etc
You will need insurance for short term lets, CCTV for security etc
Your accountant will be able to advise you on your tax affairs.