Second best would be a pair of homeplugs, one in the router and the other you take with you to wherever in the house you need the connection.
That should work in theory, but another user recently tried it and found very poor speeds, even though the powerlines themselves were capable of decent speeds in 'normal' use within the LAN.
I can't remember where the post was can can't find it again. There were various suggestions such as larger packets or a different pattern of packets; the LAN sees regular internet packets and the modem/router connection has them PPPoE encapsulated.
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BT Infinity 2, moving to PlusNet (26th Sept if all goes well)