You can sell your home at any time. But if you've never paid off any of your loan's principal, then you don't actually own much of the property.
You could still sell, but once the loan is paid off, you'd end up with very little money – especially if you had a small deposit to start with.
Once you factor in your real estate agent's commission (around 2%), lenders mortgage insurance and stamp duty on your next place, you could have nothing left at all.