Most wifi routers are already setup to bridge between the wifi and their Ethernet switch. So:

- disable DHCP server on router
- set the router's LAN settings to be on the same subnet as your house network
- plug house wifi into one switch port on the router
- plug PCs into other switch ports on the router

One thing to be wary of, if it isn't a ADSL/FTTC* router, don't plug into the WAN Ethernet port. You want the Ethernet switch ports which will all be bridged together with the wifi.

Unlike the US, most UK consumers have an ISP supplied router with one or more Ethernet switch ports and a socket to plug into the phone jack (and don't have an WAN Ethernet port). Whereas in the US with more people having cable rather than DSL, there are more routers with a WAN Ethernet port instead.