If it's a Halfords Cycle to Work Scheme voucher, you can definitely use it with other retailers - you just need to have the code to type in at checkout. I got mine from Tredz - The Specialized Turbo Como 3.0. I LOVE IT - got it in the sale as well so was cheaper. I get an excellent range (and live in Edinburgh so it's really hilly here), charges fast (4hrs from zero to full). One thing I will say is it weighs a f*cking tonne - my partner has a Trek electric and I was shocked by how much lighter his is BUT he has crap range, has to charge it every day even though he cycles much less than me, and he had to get mudguards and everything as his came as literally frame and wheels. Mine has everything - built in lights, mudguards, pannier racks etc as standard, she's a big chunky dutch bike and I love her. I used to live in the Netherlands and so very much use my bike as a workhorse - I need to be able to hop on and off in any outfit, carry shopping, kids, possibly a dog etc! His is more sporty looking because he rarely stops at the shop and is never transporting anything other than himself.
Think about whether you want a mid drive or a rear hub drive. Mid = more spendy, needs a chunky frame to accommodate it, uses torque to guide assistance which feels much more natural (and doesn't have lag). Rear hub = cheaper, not as efficient (drains battery faster), gives assistance based on cadence rather than torque which can do weird things on hills.
Don't forget to add the cost of your accessories as well, I got absolutely everything - helmet, bags etc as you get it all cheaper!