Should be just within budget. The Defy frame is worthy of a bike costing twice the price (same frame came top in Cycling Plus budget AND intermediate sportive bike test). If money is tight I'd take a Defy 3 with sora over the higher spec Tiagra groupset on another bike, as the frame won't be anything like as nice.
Defy's are great (I've owned one, loved it) but that's over egging the pudding slightly. The Defy is a good example of a very well thought out, high quality bike, from an enormous manufacturer with lots of experience doing this sort of thing. Plenty of other bikes out there are almost, if not quite as good as, the Defy for the money.
What Bez said is important though. If you have to spend hours on the bike, day in day out, fit really is the most important factor. For £800, you're likely to be looking at ally framed bikes with perhaps a Carbon fork, Sora or Tiagra groupset (or very possibly Campag Xenon), and a 9-9.5kg bike weight most likely. A Giant Defy is only the best bike if it suits you best. A Specialized, or Trek, or any host of others could well suit you just as well.
Oh, and if £800 is the all up budget, save £100 or so off the price of the bike readily and buy yourself some top notch comfortable clothing to wear on the ride. Your MTB pedals and shoes will be perfectly OK for now, so spend the money on quality padded shorts in particular, and a saddle that you can spend days on happily.
FWIW, groupset wise, now Sora has gone to the "proper" STI shifters rather than how it used to be with the smaller lever on the top, inside of the unit like on Campag, it's actually pretty good quality stuff. In fact it's pretty much the same kit as 9spd Tiagra was, just with a makeover, and if anything it's probably more reliable than the 10spd Tiagra is as I've encountered a few problems with reliability on newer Tiagra (older 9spd stuff was solid as a rock). Just my own experience, but worth considering perhaps.
Oh, and personally wouldn't spend money on a Ribble or Planet X, but that's cos I have hang ups about cheap carbon frames. That said, both brands do make ally frames that have a reasonably good reputation too, so may be worth considering, if slightly dull to look at.