The reason I suggested a stand alone business is perfectly simple. Whilst you can operate two disparate business within the same envelope you are not permitted to mix profit and loss between them. By running a separate set of accounts any problems of this nature simply do not arise.
I agree, the capital allowance (CA) system is a nonsense, it's like something out of Alice in Wonderland. The answer is you can't, so there! Here is HMRC guidance regarding buildings acquired and let for residential purposes:
'Leased out dwelling property - restrictions
Note that if your business is an ordinary UK property business or an overseas property business you can't claim capital allowances for expenditure on plant or machinery (including those that are fixtures or integral features), for use within a dwelling house that you rent out. However, expenditure on plant or machinery for use within common parts of a building that contains more than one dwelling may qualify.'
A business property is quite another kettle of fish, but dwelling houses are not, for CA purposes, business properties. It would not matter if you acquired it through a company, the same CA restriction would apply. The only way the purchase price is offset is on the sale of the property when it, plus improvements, is allowable in the Capital Gains Tax computation. Always bear in mind Benjamin Franklin's dictum that in life there are but two certainties, death and taxes.
Sorry to have to rain on your parade. Please be so kind as to rate me before you leave the Just Answer site.