If you do not make a nomination, the question of which is your main residence will be determined on the facts.
This will normally depend on a combination of factors including which one you spend more time in, proximity to work and children’s school and which address is used for bills, doctor’s registration, car registration etc. However if you make an election as to which is your main residence you are perfectly free to choose any property provided it is used as a residence by you.
Because you did not define which property is your main home , HMRC would look at the full time of ownership and where you typically had as your main address.
If the lower valued property has been used then they would need to move into the higher valued property for longer than you have lived in the lower.