Hello Khachik, I am Keith, one of the experts on Just Answer, and pleased to be able to help you with your question.
Firstly, when you left the UK in 2012 did you complete a Form P85 and send it to HMRC? If you did not you should do so immediately, fortunately there is no time limit as to its submission, it is available on the web and can be filed on line. On receipt that Department will classify you as non resident. On your return to the UK you should advise HMRC by letter of your return and the year of return will be split into two parts, one resident and the other non resident.
1. No, you were non resident and the Singapore income is not taxable in the UK.
2. It really does not matter as the split year procedure which I explained above will protect you. You will be subject to UK taxation once you are classified as resident on your return.
3. These are not taxable in the UK, but I always advise that it is safer to receive these funds before you leave Singapore whilst you are still classified as non resident. You should warn your UK bank of incoming funds and their source to preclude any money laundering inquiry a large transfer might otherwise attract..
I do hope that you have found my reply of assistance.