hi mat .i am a carpenter,joiner have been for 18 years. You say you've been training for 3-4 years, have you passed any exams yet? Don't take this as gospel but i think, going by what my agent told me you need 7 years experience within your trade, if you don't have the relevant paper work, but again don't take this as gospel.
There are tons of people on here who are more informed than i am. As for getting your skills assessed go on the
Department of Immigration and Citizenship (i think this is the name - if this is wrong i'm sure someone will have corrected me, if this is wrong) you'll get all your advice there.
Assessment cost is £300 which is non-refundable.