Under the government's Rent a Room scheme, Hosts can earn up to £7,500 per year from rental income before having to pay tax on that income. 

There are, as you would expect, some stipulations and reading the rules on this would be a good place to start. 

Still, it's a nice incentive – if you only let one room @ £100 per week you can enjoy 52 weeks per year of happy, paying tenants without paying any extra tax!