Central Heating forms part of the Fixtures and Fittings as far as capital allowances go if you're claiming for the building itself.
In relation to the question about calculation, search Google for "property capital allowance indicative savings'
Exactly. The question is "can you take it with you?". Clearly, in relation to a central heating system that's not likely. So with a trade, you're in IBA territory. In a rental income scenario, I'd treat it as enhancement for CGT purposes to be included when the property is disposed of.