Mod,
You seem to refer to capital losses & capital allowances in your two contributions so far. These terms aren't really interchangeable
Capital losses are basically losses arising on the sale of a capital asset. So if you sell some shares at a gain you have a capital gain and pay tax accordingly. If you sell shares and have a capital loss, no tax is due and the loss can be set against capital profits on other assets in same period or carried forward indefinitely.
Presale expenditure on capital items e.g. new roof on house before selling represents enhancement expenditure and can be deducted as a cost of acquisition and indexed if applicable
Hope this explains