To pay for the increased mortgage when I bought my flat in 1991 I started letting rooms in both properties. At some point, I cannot be certain if it was within 2 years, I declared my house as my main property to qualify for rent-a-room there. I still let rooms in my house now.
More complicated than I thought!
I bought the flat to be near my College, however every weekend, half-term and holiday I stayed at my house, to be with my son who normally lived with his mum after she moved out. My time was shared almost evenly between house and flat.
About 5 years later when sorting out tax due on rooms let, I successfully argued that my house was my main residence, and have been using rent-a-room since, although I've yet to submit my 2013-14 tax return.
I hope this clarifies my situation.