Tax treatment of repairs in change from furnished to unfurnished lettings

Refurbishment
The cost of rewiring, new heating system, kitchen units etc are capital expenses and would not form part of the rental expenses. These expenses would be used against capital gains tax when the property is sold.

Don't entirely agree

You are correct that there is now one property business and the cessation of letting furnished and commencing letting unfurnished after refurbishment doesn't affect the recovery of revenue expenditure incurred wholly and exclusively for the property business.

What it does affect is the ability to claim for renewals of furniture or the wear and tear allowance. Both of those relate to furniture though, and your questions concern fittings.

The kitchen units are more likely than not a repair (unless there's a significant level of improvement), and this is confirmed in the recent draft guidance issued by HMRC under cover of R&C Brief 05/13, which might also cover the new heating system (again dependent on extent).

Rewiring may well also be regarded as a repair to the entirety of the property (again subject to the extent). There is case law somewhere though where the complete rewiring of a hotel was, as suzytoots says, regarded as capital.

To the extent that the items are capital, I'd agree that the only means of gaining relief is against any capital gain on disposal, and then only to the extent that the expenditure enhances the value of the property on disposal.