If you go by Apples other products at the same pricing:
IPad Pro starts from $799
The base model Mac Mini with 16GB Apple ram is also $799
In the UK the Mac Mini with 16GB Apple ram is £639
The iPad Pro 128GB WiFi is $949
The iPhone 6S Plus 128GB is also $949
In the UK the iPhone 6S Plus 128GB is £789
The top range WiFi and cellular 128GB iPad Pro is $1079
The top of the range Mac Mini with the Magic Mouse 2 is $1078
In the UK the same top end Mac Mini and Magic Mouse 2 is £864
That should give you a rough idea I guess? I won't be surprised if you can add on an extra £30 to the top two iPad Pro prices I've put here.
I think you're numbers are pretty much spot-on, although I'm not sure about price comparisons across product categories.
Look at US$ prices within the same product family then use the same percentage jump:
Base-line iPad Air 2 (16gb) is $499
Base-line iPad Pro (32gb) is $799
That's about ~160% jump
So
Base-line iPad Air 2 (16gb) is £399
x ~160%
Gives ~£639 for the Base-line Pro
Double check with existing product lines for comparison:
iPad Mini 4 (16gb) is ~80% price of iPad Air 2 (16gb) in both US$ and £.
Apple TV 4 (32gb) is ~217% of Apple TV 3rd Gen in both currencies.
Use the same formula to get these approx iPad Pro prices:
iPad Pro (32gb) £639
iPad Pro (128gb) £759
iPad Pro (128gb+Cellular) £859
We'll know in a few days hopefully and don't put it past Apple to 'round up' the prices a tad. I'm fully expecting to be wrong.