The difference between a private trader and a unit trust is that unit trusts are exempt from CGT but pay a flat rate of income tax at 20%. The other big restriction is that Unit Trusts cannot borrow money so could never open a leveraged position and thus trading forex is a non starter for them.
The same rules apply to both though as regards to whether a disposal is an investment and subject to CGT or whether it is a trade and subject to income tax. If you have short holding positions (days or hours) and are a high frequency trader (a few every week) then your disposals will likely be part of a trade and subject to income tax not CGT. There are several other conditions that HMRC look at in deciding whether a disposal is subject to CGT or is conducted as part of a trade, but I can assure you apart from not employing sales staff or advertising the goods for sale, the rest of them pretty much nail Forex trading as being classed as a self employed trade.
As self employed trading income you will also be liable to Class 2 and Class 4 NIC which are £2.50 per week and 9% of profits between £7,225 and £42,475 and 2% of all profits above £42,475, in addition to income tax at the normal rates. You will not have to declare all of your trades on your tax return but you will have to declare the total P/L for your accounting period. The other nasty surprise will be that you will effectively pay 150% of the tax due the first time you declare profits as 100% will be the tax due and 50% will a a payment on account (due on the 31st January in the year after the tax year in which you made your profits has ended) with a further 50% payment on account due on 31st July of the same year.
I have had many dealings with HMRC and I would never trust what one person has told me on the phone. I have asked the same question 3 times to 3 different people on the same day and got 3 different answers. You will not be provided with a reference number or the name of the person you spoke to HMRC and their opinion is not legally binding. Your only hope would be to get it in writing from them but good luck with that!
If you really want to find out from the HMRC, then submit a tax return classing all of the profits as being subject to CGT, but to do so you will have to submit your calculations for each gain and loss which means detailing every single trade. Your continuation sheet is likely to be several pages long and all tax returns get the quick once over for obvious errors so this may or may not generate an enquiry. To open an enquiry HMRC have 12 months from the submission of your tax return to start one, although the time is unlimited if they think you have been negligent or fraudulent. Just because it wouldn't get flagged one year though, it doesn't mean it's ok so you would be waiting to see if they ever opened an enquiry. If they did they could backdate it, and if they thought you owed them unpaid tax then they can impose penalties, surcharges and interest would could bump up the underpaid tax you owe them by as much as 50%. At the moment HMRC are understaffed and are taking forever to even acknowledge post, so they might not have the resources to pick up on it, however, they are being extremely ruthless with any unpaid taxes so if they did pick it up then watch out.
This all refers to trading Forex through CFDs with a Forex broker, so if possible I would try to use a spread betting account because as long as it is not your only income 'casual gambling profits' are exempt from both CGT and income tax. As a rule of thumb you would need another source of taxable income that could cover at least your basic outgoings such as rent/mortgage, bills, food etc even if the taxable income was considerably less than your trading income. A good way to achieve this is to pay your mortgage and debts off asap from trading profits and thus reduce the amount of money you need from a taxable source.
what about setting up an offshore comapny?