Harry,
This is a very difficult area made more complicated by subjective and inconsistent decisions by various local tax offices who often don't really understand what we do. Also, I doubt there's any distinction made between 'trading' and 'day trading', although arguably the latter carries even worse psychological baggage than the former. As for 'registering' as a trader - I'm not quite sure where you're going with this. At some point it may be recognised as what you do but you don't need anyone's permission to start.
I think most traders would prefer to pay Capital Gains Tax while HMRC would much prefer to charge income tax and this is one of the points that can get quite complicated. Generally, it's accepted that equity and CFD gains are subject to to CGT, but if it's your main source of income, they might try to impose income tax on you.
Also, there's the question of what you're trading. If its spread bets, there's no tax on this activity with the caveat that they might retrospectively try to apply it one day, and you don't have to tell HMRC anything, except if it's your main activity - in which case maybe you do and maybe it's regarded as a job with a taxable income. If all this sounds rather woolly it's because it really is a minefield.
If you're a subscriber, there's a good thread in the Traders' Room on these issues here:
Minimising the tax burden for full-time traders
This newspaper article may be of interest:
Taxman faces new criticism on active traders
This site may be of some help:
Tax Cafe
And this is the accountancy company which quite a few of the traders on MoneyAM use:
Trio tax for financial traders
Everyone's circumstances are different and if you're getting into this seriously you'll almost certainly want to take professional advice, probably from Trio because your local accountant won't have a clue. I know of someone who used a local accountant who checked things with the local tax office, got bad info, ending up in a disputed tax filing which led to an investigation. Moral of the story - find a real trading tax expert.