For the accounting treatment, just google FRS10,,,
Edit, just reread your post -- tax treatment is not governed by standards, i dont have a link here right now, but it should be somewhere in the tax consolidation act
Edit 2
from the notes on the back of a CT1, corp tax return
2.51 Depreciation, Goodwill / Capital write-off – Depreciation is depreciation of business assets provided for during the
accounting period. It should be added back in the adjusted profit computation. Goodwill /Capital write-off relates to any
write-off of the value of assets during the accounting period. It should also be added back in the adjusted profit computation.
treatment is similar for a soletrader I imagine, but someone else might confirm