So the reg fees will be class as stock or asset and so it won't be realised until a sell?
You don't own them, and neither do Nominet, who consider them akin to a telephone number (a stance soon to be re-enforced with the introduction of the ENUM system, how nice!).
Reg fees are an investment cost. If you develop a site to earn revenue from, the set-up cost is also an investment cost. Revenue from Adsense, affiliate marketing, etc are liable for tax against the business profits, or a straight forward taxable income for the non-limited self employed, offset against costs.
A one off genuine private sale of the domain name can be offset against CGT, the remaining (if you had a big enough sale) against earnings.
If registered intending to profit, and in particular with multiple registrations as a straight business activity, IR only interested when you sell, as a non-declare is evasion. Only my personal take on it, of course, you'd need to take proper advice!
Remember, the IR have long arms if you don't pay your tax liability, and they can chase you for non-payment going back many years. Once they have their teeth into you, they have a habit of not letting go.