Paying tax on rental property in UK as a Canadian resident

We left the UK in 2013 and have rented our apartment to friends and family ever since.
We make almost no money from the rental, it barely covers costs and have had to pay out for repairs since we have left the UK. We were unaware we would be liable to taxation as we make no money from the rental.
Now I’m told we will be liable for taxation and fines for late filing. What should we do next?