I presume anything fitted (i.e fitted wardrobes/sliderobes) or a fitted study, would be allowable as a capital cost, as it becomes integral to the building?
Not necessarily. To be allowable such expenditure has to be capital in nature and incurred on the asset for the purpose of increasing the value of the asset, and be reflected in the state or nature of the asset at the time of disposal. Would fitted wardrobes or a fitted study increase the value of the asset? Possibly. But they could equally reduce the value of the asset.
The allowable enhancement expenditure argument is fairly simple with loft conversions, extensions and conservatories. A greatly improved kitchen or bathroom should be treated as enhancement expenditure. But I don't think it's so clear cut with something like a study or bedroom. I suspect we agree that £100 worth of IKEA units/desk fixed to a study wall would not be allowable enhancement expenditure but a fitted £3,000 office suite would be allowable. Where is the line? When does (or doesn't) fitted units/furniture become part of the premises?