Having only the one HDMI lead might not be a problem. You can always stick a switch on the end to select between sources if necessary. And of course if you use a surround sound system of any reasonable quality it will have multiple HDMI ins for the external kit and only a single HDMI needed to the telly.
Speaking of which...
If there is no surround system at the moment it might be worth checking if the TV supports Audio Return Channel over HDMI. If it does then run a cable to that socket. That way you are set up for any future addition of a surround system. Just get one that also supports ARC and you don't get the "DOH!!!" moment as you realise there is no way to get sound out of the telly into the surround system for those occasions when the built in Freeview is in use for example. Particularly so if all the cables are buried in the wall