You can't really get a contract via an intermediary (an agency) as self-employed, as they're liable if you don't pay tax. Having a Ltd also allows you to defer

You can't really get a contract via an intermediary (an agency) as self-employed, as they're liable if you don't pay tax. Having a Ltd also allows you to defer paying higher rates of taxes, avoid paying NIC (both employers and employees), and I'd say it's simpler being two entities.