Hello and welcome to the site. Thank you for your question.If you have received a notice to file a tax return then you must file it as failure to do that would result in a fixed penalty of £100. You should file a tax return albeit as a NIL return.I am not sure if you filed a P85 form when you left the UK. Filing of this enables HMRC to update their records and get your tax right. You can find more information on Form P85 here
You state "I was living in Ecuador and I made close to £17000 by speculating on-line on bitcoin markets. I withdrew all he money to my UK bank account because i owed tax from 2012, so although I was not in the UK, all this money went to my UK bank account by selling the bitcoins I speculated on to UK individuals via our personal bank accounts."If you are non-resident in UK for tax purposes throughout the tax year then any gains you make from investments in that year are not chargeable to CGT in the UK. You say you have been out of the country for the last 3 years and if the buying and selling of bitcoins took place whilst you were non-resident then the gains are not subject to CGT.
It is your residence status and not where the bank account is held that determines whether a gain is chargeable to CGT or not.
More information on this can be found here - look at Q4
There is some useful information of HMRC helpsheet hs278 "temporary non-residents and capital gains tax" here
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.
I have yet to finish reading all the links you posted as they are quite long, but just 3 quick questions so that I can properly formulate an understanding.
1. Since CGT occured outside UK, I can simply file a NIL return, correct?2. I have to fill out a P85 form now.
3. Arbitrage gains are handled as capital gains tax also?