Hi Caz, Welcome.
Echo much of the above.
Compact frames have been the rage for a while, but don't get hung-up on that, they have a benefit in that the sloping top tube makes the 'stand-over' height lower.
Compact Chainsets have recently come into vogue too. They're like Marmite, you either love them or hate them. One of my riding buddies bought a Bianchi with a compact set-up and whilst he was OK with it initially, he now hates it with a passion, the big gap between the front cogs can make for a lot of double shifting. Personally I prefer a triple, I can see no real disadvantage for a rider of my abilities and I get a nice wide range of gears that sit closely together with easy shifting patterns. If from an MTB background you're used to a triple anyway. Considering my other bike is a fixed-wheeler I guess I have the sublime and the ridiculous gear wise.
£500 to £600 buys a lot of bike these days, usual suspects are from Trek, Specialized, Giant, Bianchi, all offer good vfm.
That price range there will be doubles (53/39T), Compact Doubles (50/36T) and 'Road' Triple (52/42/30T or 50/40/30T) options. Bikes will be Aluminium with Carbon forks, some may well have Carbon rear triangles as well. Component-wise look-out for Tiagra upwards with Shimano and Mirage and upwards from Campagnolo (goes from Xenon, Mirage, Veloce, Centaur, Chorus, Record). What's to choose? Well arguments will rage, but, themajor difference is the feel of the shifting/brake levers. Campagnolo and Shimano both have slightly different shift mechanisms, Shimano lever hoods are more rounded and comfortable, Campagnolo are neater and all the cabling is hidden so is better looking. Basically, try both and see what suits, they both do a good job.
Weak points on this price of bike is usually wheels and tyres which tend to be solid and reliable, but heavy. Wheels and tyres are usually the first and most cost-effective upgrade.
Few road/race bikes will take full guards for all-season usage (race-blades can be a fiddle and only do half the job...I use my TCR all year-round (not commuting) and just get wet!
The Giant SCR range and The Trek 1000 will both take full guards and a rack for light loads and also come with Triples. Both are good all-rounders. The Specializeds and Bianchis are a bit more racy and neither take guards or a rack AFAIK. The Giant SCR also has a less extreme riding position by virtue of slightly higher bars (though they can be lowered a fair way if racing), useful if slightly less flexible as some young striplings (am 43 btw).
Finally, bike fit is very important, some bike shops will do a proper measuring service at around £30 and I would say it's well worth it.
Don't get talked into having to have a WSD or Woman Specific Design unless you are particularly petite. A bike either fits or it doesn't and (whilst many will tell you different) on average women are proportioned the same way as men with the exception that there are more small women than there are small men...if that makes sense. All the women in our club ride regular bikes without issue.
Finally Finally. Go to alot of shops and ask questions, be happy with the answers, if unsure, ask here. ALWAYS test-ride the bike of choice, set-up in the right size before you buy.
Then the grin is just a credit-card away...