try one of the new isps e.g BE INternet - they put there own kit in the exchanges as does Easynet (double check with the latter)- get a pix firewall enable a feature called ISP Backup - and away you go.
resilient
if ur in 0207 radianz will give u complete resiliency provided you have colt and bt in ur building i.e. u'll get 2 lines as standard and access to a financial community spread acros the globe consisting of some 15000 connections