Wtc last year income much lower than this year...

Our income this year has risen to 18.5k, husband works 40 hour week, last years income was much lower only £7.5 k and hubby was self employed for most of the year and was running at a loss, so will our wtc be based on this year rather than last year as there is a big difference?

I don't understand the whole £5 k disregard thing and also what the threshold for watch is for a joint claim... Can anyone enlighten me on what we will get this year?

Thank you in advance :)