The amount of the loan has to be secured throughout the term as a first charge on any asset owned by the person borrowing the money. This asset must be worth at least the same as the amount being lent plus the value of all the interest due over the term of the policy. If the asset that is being used as security is replaced by a new asset, the value of the new asset has to be at least equal to the value of the outstanding loan, including interest. In practice the security is likely to be a property.
HMRC Pensions Tax Manual - PTM123200:Investments: loans: loans to sponsoring employers