Left UK with small amount of debt

I left England with a small amount of debt (£300) as I didn’t pay the remainder of my broadband contract with Three. I don’t see myself coming back to the UK as I’m not a citizen. I do still have bank accounts and an ISA account that I have not made any deposits into since I’ve left. They’ve just sent my debt to a collections agency and I’m wondering if they’ll be able to freeze my bank accounts or chase me down for it?