Companies can be dormant (inactive) for lots of reasons, such as:
To protect a company name
To reserve a company name for later use
Death or long-term illness of the owner
Restructuring of the business
Owner is taking a break for personal reasons - pregnancy, travel, study, employment
For the purpose of being a second member of a limited liability partnership
A company can remain dormant for any length of time, as long as it does not receive any income or have any transactions in its bank account other than payments from the subscribers for their shares, Companies House filing fees and late filing penalties imposed by HMRC.