First, I'm with umazuki. Tell the dealer you're looking to spend around £600, then work your way from there. Remember accessories will eat up that remaining £400 very easily--lights, rack, mudguards, panniers, lock, repair kit, pump, helmet, lock(s), raingear, etc.

Second, for five miles each way, you can ride darned near anything. At that point it's not the distance or roadway conditions that dictate the bike, but rather, your personal preferences, your commuting style, and available facilities at work.

For example, showers, lockers, secure bike storage (locker or room), and wanting a workout on the way to work point towards one type of bike. No showers or lockers, a bike rack on the street, and preferring to commute in work attire dictates other choices.

Finally, are you already a cyclist, or will this be a first bike? If the former, let your experience guide you.

If the latter, bear in mind that the purpose of your first bike is to teach you what you want and need in a second bike. Therefore, expect that what you learn about cycling and about yourself as a cyclist will mean you'll want a different bike in six months to a year. Plan (read: budget) accordingly for your first bike.

My first bike was an inexpensive hybrid. I planned on riding 2-3 miles each way to work, 3-4 times a week in work clothes, and never in foul weather. First I learned that I sweat profusely no matter how cool the temperatures. Second, I learned that I really, really, liked cycling and that 2-3 miles wasn't enough for me. I also learned that I wasn't content to pootle along at a processional pace. I liked riding fast. I was surprised to learn that not only didn't I mind foul weather cycling, but that I thoroughly enjoyed it. And finally, although my workplace (a public library) is one of the few places you can reliably find bike parking racks in the US, the boss insisted I park my bike indoors.

Nine months later, I had a second-hand road bike with mudguards, rear rack with panniers, and lights. I rode to work in cycling gear carrying my work clothes in the panniers. (The branch I work at now has lockers, so I haul everything in on Monday, then ride any old bike I want mid-week, and haul everything back home at the end of the week.) And I began something I continue to this day, taking the long route to work. I currently cover four times the distance of my shortest route, just because I like it.

Things will probably turn out differently for you than for me (or anyone else), but my point is that until you're doing it, it's tough to predict what will work out for you as a commuter.

If you already know (not suspect or speculate, but know) some of these things, then that can help us narrow the choices for you.