Hello, I am Keith, one of the experts on Just Answer, and happy to help you with your question.
I am of the opinion that your assumptions are correct. Your buy to let properties aggregate to form your trading capital and operate as a block so all refurbishment expenses are pre letting costs and can be claimed against the property income. Remember that property profits and losses cannot be offset against you other sources of income, a small point, but one worth remembering.
I do hope my answer is of assistance.
I assume using a single property for some other use "temporarily" does not affect your opinion in particular that the expenses should be wholly for the business.