Picking a bike for a cyclist new to riding on the road? I've gone a few years without riding at all, and I'm getting back into cycling to improve my fitness. I've typically ridden light mountain bike trails in the past. I want buy a new/second-hand road bike for commuting but I don't want to spend a huge amount of money. I'm still new to riding on the road, and don't know if it's something I'll be able to keep up.

Recently I was told by someone I know to be a proficient rider not to spend any less than £500(GBP) on a road bike if I wanted anything worthwhile. However, I don't want to become a typical "all the gear, no idea" by blowing all my money on a bike that is way beyond my needs or ability. My problem is picking a bike that matches my needs and ability with limited knowledge of commuting by bike.

I guess you could translate my question to "What should a new road cyclist look for in a bike?"

For example: With a little shopping around I've seen this bike which is appealing to me: Woodworm White Lightning Road Racing Bike  Would that be suitable?
My usual answer to the question of "What sort of new bike should a (relatively speaking) novice cyclist get" is "used".  Better to ride a year or two on a moderately priced used bike than to drop a bundle on the WRONG new bike.  

(Nothing wrong with your "light mountain bike", BTW, other than you'd want smoother tires, the suspension may be too soft, and maybe your bike can't mount a rack very well.)

Eventually you need to decide:


Straight bars or drops?
How many gears?
Rack?
Fenders?


The answers to these questions can only come after you find out how often/far you're going to commute.  You may start out loafing along and discover that you like to sprint all the way (meaning drop bars would be a good idea).  You may start out a fair-weather cyclist and get so into it that you're riding in the rain (where you'll really appreciate a bike with sufficient wheel clearance for fenders) or in the snow (where your current mountain bike, with studded tires, might be just the ticket).  You may start out cycling just a half-mile each way to work, then discover that you like to take "shortcuts" of a few miles.  (I ride 10 miles to work, but commonly take a "shortcut" in the morning that adds another 25.)  You may discover that you bike to the grocer and to the theatre on weekends.  You may carry a change of clothes in a small fanny pack, or you may need panniers to carry clothes and laptop/papers for work.

Or you may discover that you don't really like cycling that much and just do that occasional half-mile jaunt.

And it's also important to get a bike that FITS well, especially if you'll be riding any distance.  This means the right height for your leg length, the right top bar length for your torso length, an overall geometry that suits the flexibility of your body (understanding, BTW, that excess avoirdupois significantly limits ones ability to lean forward and low on the drops).  Some of this a good bike shop person can help you with, but to a large degree you must learn "fit" from trial and error.