Can I be dismissed with notice with no disciplinary hearing and suffer from depression?

Whilst visitng family overseas on annual leave I was signed off for 2 weeks for depression.  I stayed in the coutry as I had holiday booked over Xmas.

Returned to work, invited to a meeting that day. Interogated, told I had lied as hadn't intended to return to UK. Next day invited to an investigatory meeting where I was dismissed with one weeks notice (have worked for 21 months). Am I not entitled to a month?

I was not allowed to be accompanied and was not asked for my evidence (return ticket/pre-booked parking/singe ticket to UK).