As far as I know, as you are going to take the exam as an outside student, you will be able to do the test with one of the institutions that take outside students. Probably AQA and Edexcel are the most "user friendly". You need to contact them asap, as entries for this years GCSE maths are done kind of by the end of January. When you enter yourself, you will have to say if you want lower or higher tier. I would recommend you do the higher tier. It´s not just that the lower tier is easier, you can only get a fail (which let's be honest a D is) or a pass (aprobado) which is a C. The higher tier is not that much more difficult if you actually study a little bit. Nowadays in the UK, there are many questions in the Maths tests that /are about common sense rather than Maths, and you will find that it´s not really as difficult as ESO/BUP. ---- Probably, also a good idea to buy a Maths revision guide online and get it shipped to Canada. Go to the sites for the examination board and download some of the old papers and get somebody to help you do them.