There are additional circumstances where a tax return will also be required or hmrc will at a minimum need to be notified of details.
Sometimes you need to report to HMRC even if your gains are below the tax-free allowance. Send a tax return if:
you disposed of chargeable assets with an overall worth of more than 4 times the tax-free allowance - this works out as £43,600 for the 2013 to 2014 tax year
you have losses that you want to claim (if you’re not already registered for Self Assessment, you can write to HMRC instead)
https://www.gov.uk/capital-gains-tax/wo ... eed-to-pay
Note hmrc are unlikely to agree that you are self employed if that would suit you tax wise, however if you applied to be self employed that application would likely be accepted and they may only later question the status when it suits them (Eg if you claimed losses or didn't declare capital gains)
Note the more you can trade within an ISA the better as the reporting headache for shares is quite high. Anything within an ISA is ignored from a reporting point of view.