What is the best psu for under £100 that is 850w, or higher and supports sli?

Rammy

Honorable
I would absolutely go with one of the XFX 850W PSUs for those criteria.

£80 - 80Plus Bronze - http://uk.pcpartpicker.com/part/xfx-power-supply-p1850snlb9
£90 - 80Plus Bronze Semi Modular - http://uk.pcpartpicker.com/part/xfx-power-supply-p1850xxxb9
£100 - 80Plus Silver Semi Modular - http://uk.pcpartpicker.com/part/xfx-power-supply-p1850bnlg9

All are good, I'd probably go for the middle one as it's probably worth an extra tenner.
There are alternatives from Corsair, Seasonic and Antec but all come in at over £100.