I've got a 4G USB modem on the shelf, for backup in case of BT living up to their reputation. While I've tested it briefly on my desktop, I need to do a forklift upgrade of the router hardware before I can install an OS that supports it.
Plan is to do L2TP failover (using a Three PAYG data SIM, because cheap) for the ADSL connection, so not too fussed about performance (laggy internets are a lot better than no internets, and we can forego streaming video during a house move or our-favourite-telco incident). I expect it will be adequate as we're in an urban area with plenty of 4G, though I expect the latency to be underwhelming for interactive shells or gaming.