1.If you've got the offer use it, they might take it away otherwise and others might not be able to gain advantage from it then.
Halfords at least put money back into cycling, sponsoring a road team and a few xc people.
2. what sort of riding do you do and do you have any desire to try other things?
e.g. they do the Carrera Titan Carbon for £1000
Carrera Titan Carbon
I'm sure you could find something there just as an alternative bike – bigger, faster, winter, summer, race etc option
3. It gets great reviews as a road bike and if you think you'll use it, even occaisionally then it's a good choice.
4. We're not your mum, although admittedly it's not permission you want. Just get something a bit different to what you would normally get.
If you're a member of British Cycling you get 10-20% off at Halfords although I don't know if would work in conjunction with your voucher.