I'm in the UK and have just purchased a 3 pack of the powerline P9 deco units and have the same question.

The UK power cables look to be standard fitment (3pin UK plug on one end and what looks to be a standard appliance C7 style 2 socket end at the other end that plugs straight into the P9 unit).

I suspect as long as a decent quality longer replacement cable is used the powerline capability should work as before. After all there is a lot of variability in the length of the wiring from the wall socket via the consumer unit to the other deco units  (from a powerline perspective). SO an extra metre or two on the power cable to the unit seems unlikely to affect the performance.

The cables are pretty reasonably priced, so I will try a 3m metre one on one of the units to see if this affects the performance and post back here.

If you look in the Deco mobile app on iOS and view the details of each non-master unit, it shows you how it is connecting back to its nearest unit. Usually for the P9 units it will show the dual wifi bands and signal strength for connection back to its nearest P9 unit. Also if it is using the powerline cabability you will see a "signal pulse icon" with PLC next to it.

I think this means the unit can connect back using both WiFi frequencies and the powerline connectivity. The web interface might give slightly more details on the state of the connection.

Would be interesting to hear from the TP Link team on the power cable though, and if there is anything special about it compared to normal C7 style power cables used for other home mains powered devices (assuming you use the same fused capacity in the plug).

@Raeine 

I bought a 3m cable from CPC-Farnell, specifically, this one:

cpc.farnell.com/pro-elec/pe01056/lead-uk-plug-to-fig-8-c7-white/dp/PL13302

(can't post it as a proper link for some reason)

Only problem was that the C7 connector is a bit larger than that supplied by TP-Link, so I had to cut away (very carefully!) some of the strain relief before it would fit into the socket in the base of the P9.

@Chris_B got a link to the ones you purchased?

@timp78 - I posted a reply with the amazon uk links in it, but I don't think the forum software settings allow links as it hasn't appeared sadly?

Just choose any of the higher feedback rated uk plug C7 cables in the length you need and that should work. I used 2 different brands to get the lengths I needed and both worked without issue.
 

Thanks