I'm also that length ( 3.5km straight line) from the exchange, my line length is actually just under 6km..
I had Sky installed to test the speed though I didn't plan to use it long as FTTN (HFC) was available but wanted to get an idea.
The Openreach engineer who came to activate the broadband got the line length for me and told me that it was probably one of the best quality lines in the UK he has ever connected...
I was estimated 1.9-3mbps download, I got 2.08mbps and very stable.. But cancelled it after 30 days as I didn't plan to keep it.
My estimate now via Sky and BT Wholesale checker has dropped from 2mbps to 1mbps (0.5-3.0mbps)
I would estimate that you would probably get 1Mbps and as already mentioned if the line is good.
Different providers vary though..
Its a gamble I'm afraid.
Do keep me informed on what you get when installed as I would be interested to compare.