I would have recommended a 550w psu but looking at reviews the 1080ti on it's own can pull up to 339w (depending on the card you buy) and that's without overclocking it so I would be looking at a 600-650w unit.
My basket at Overclockers UK:
These are all fully modular, gold rated psu's. The EVGA, Seasonic, Superflower and XFX all have a switch to change between normal and Eco/Hybrid fan mode. This mode means that the fan only starts up when the psu hits a certain load or temp. The Bitfenix and Corsair both have a quiet fan.
The Bitfenix Whisper M 650w is a CWT built unit with quad 12v rails and the full 650w is available across them. Each rail delivers power to a dedicated component group so, 12v1 is motherboard and peripherals, 12v2 is CPU, 12v3 is GPU 1, 12v4 is GPU 2. All Bitfenix Whisper M series psu's have a 7 year warranty. I included the 750w version as it's such good value for money. The one downside to this series is that the 24 pin cable has a shroud over it which you can see in the pictures from
this review. If you don't have a window on your case it's not a issue.
The XFX XTR 650w is a Seasonic built unit with a single 12v rail of 648w. It has a 5 year warranty and you can read a review
here.
The EVGA Supernova G2 is a Superflower built unit and is based on thier own Leadex Gold series but with no annying led's, a different fan and a few internal tweaks specified by EVGA. It has a single 12v rail of the full 650w and has a 7 year warranty.
Review here.
The Superflower Leadex II is the latest version of their Leadex Gold series and finally has a switch to turn those awful led's off. It has a single 12v rail of 649w and has a 7 year warranty. There are no reviews yet as it's too new but you can take for granted that they have taken a already cracking psu and made it even better. The New EVGA G3's are based on the Superflower Leadex II as well.
The new
Seasonic Focus Plus 650w has been getting very good reviews and are competitively priced which is a pleasant change from Seasonic. They have a single 12v rail of 648w, are very compact units at only 140mm long and have a 10 year warranty. You can read a review
here.
This one is semi modular but still gold rated.
The latest Corsair TX650M is a Great Wall built unit with a single 12v rail of 612w and a 7 year warranty. It has the weakest 12v rail of all the psu's I am recommending but that doesn't make it a bad unit. In fact the new TX M series have been getting very good reviews and are very good value for money. There are no reviews of the 650w version yet but there is one
here for the 750w version.