It varies depending on which of the very many cycle to work schemes that your work has signed up for.
The last place my OH worked for, he was restricted to Halfords only.
The last place I worked for, I was restricted to Evan's Cycles and only had to say how much I wanted to spend.
The current place my OH works for appears to be much less restricted and you simply name the place and bike and they purchase it direct.
I have to go to Evans Cycles, its the only company we use.
For the OP's benefit - I wouldnt get your voucher until you know what you want. If you get £800 on the voucher, but end up spending £650, say, you would still be left with the remaining balance to pay