Just my two cents - never buy a bike online unless you have sat on it first!
All bikes have differing geometry, it makes a vast difference in road bikes.
Try as many as you can, ones in shops - go for a test ride.
Try any friends who'll let you too.
Make sure you get sized correctly too, as a bike too big is a barge and one too small is wrong.
I'm 6'2" and I ride a 58cm.
Quick Sizing Guide
Sit on bike, heal of foot on lowest pedal, your leg should be comfortable straight not stretched. Your seat is now at the correct height.
Get someone to hold the bike, put both feet on the pedals, hold the "hoods" of the shifter, not the drops.
Look for the hub of the front wheel, it should be obscured by the handle bar. If it is behind, bike too long, infront too short. Hidden perfect.
Unfortunately bikes are like everything in life, you do indeed get what you pay for.
But don't waste your money on all the bells and whistles, if it's your first road bike, an entry level will do. Find out if you like it first!!!
But yes £600 odd quid would be realistic.
Remember the big brands have more buying power from the likes of Shimano etc so tend to have a better all round package.
Trek, Specialized etc etc.
Once you've found one then look for it cheaper
Last years model etc, now is a great time as the bike year is SEPTEMBER to September (want to make sure new years stock is all in for Christmas). Grab a last years bargin.
(Spent 8 1/2 years of my life in the cycle trade - including being a store manager)