There is a new Tokina 16-28mm f/2.8, but it is not that great and sells for 800-900GBP. I would choose the the Nikon 16-35mm f/4.0 VR for 800GBP.
There is a Sigma 12-24mm for FF, it is also normally around 700GBP. It is not a great lenses but is uniquely wide.
I would either await to buy the Nikon of your choice, or go with soemthing like the Nikon 16-35mm, Look for any of the above second hand though and should be OK. Buying a new Sigma/Tokina lenss and selling it a year later would not be a good idea, loosing you some cash. Nikons will hold their value better, sigma/Tokina work out nicely when purchased 2nd hand,and when you can test the lens before purchasing.
there are some cheaper options out there, but a little obscure. There is a Nikon 18-35mm f/3.5-4.5 but it is pretty poor and still 400GBP. the Nikon AF-S DX 12-24mm f/4G will work on a FF sensor form about 20mm but is a bit of a waste really, still about 70GBP The Nikon 16mm prime is terrible, avoid. Sigma have a 20mm f/1.8 prime hich a nice idea but the lens fails to deliver, pretty soft and lots of variance in copies. The Nikon 20mm f/2.8 could be worth a look. The predecessor to Nikon 17-35mm f/2.8 was the 20-35mm f2.8, it is an AF-D lens and quite good. You will have to look for a 2nd hand one.
I am not familiar with any of the older 3rd part lenses but I doubt they are realyl worth parting cash for. Wide angle lenses are very hard to make on SLRs. Wide angle zooms are a modern invention really.
To really save money you can learn to stitch images To do this properly you really need a panoramic tripod head which is not cheap, or a TS/PC lens (just shift left and right to the max extent and grab 3 images, left, middle and right.)