If you just use it to go from and to work (less than 20 min) and the occasional drive to Tesco or so, I wouldn't spend more than 300 pounds on it. The more value, the more likely it gets stolen.
You can however use this voucher to also buy equipment: light, lock, helmet, bell, mud guards and so on. Ah, nearly forgot: get a 3 years free service for your bike as well! That way your company pays for service insurance so, say, you have a puncher in 8 months, you only pay Halfords material to fix it, not the labour. That's really worth doing if you intend to keep your bike for longer than 1 year!
Best thing is to go there and get advise in your local store. They can often tailor a bike to your needs - and might have local offers that safe you even more money.
... hope this could help.