New road bike under 1000 GBP

The 2 speedsters will have exactly the same frames, the numbers just denote the level of all the extra kit. Hope you enjoy the test, Scott make some great bikes

Both look like good buys for a first road bike if you're looking for a bit of speed.  There really isn't much to choose between them.  However, the Scott does compromise slightly with the brake callipers but on the other hand does appear to be set up for mudguards which look tricky to fit on the Izalco (might not seem important now but in the winter mudguards are a real benefit both in terms of keeping you clean and also protecting the bike and would also mean this could then become an excellent "winter bike" should you eventually upgrade).  I wouldn't worry too much about the weight difference, it's nominal, weights are often claimed inaccurately anyway and it's likely that the difference lies in the wheels and could be improved with an upgrade.  I don't know either set of wheels so it might be worthfinding more out about them (weight and hub quality are the factors I'd look into).  Wheels are one of the most important components and often where compromises exist in mass market bikes.  The Scott also comes with 28mm tyres which I would say are preferential for general road riding but again this is only a subtle difference and can be easily addressed when you replace the tyres (both frames appear to accomodate up to 28mm although the Izalco comes with 25mm).  If your buying through a LBS and prefer the Izalco, you might even be able to pursuade them to switch the tyres and cassette.  Again, the difference in largest sprocket size is not that significant but clearly, if you're going to ride hills the 32T is preferable (although gearing is a matter of personal choice depending on your physiology, strength and riding style).  Again, you will need to replace the cassette in due course due to wear.  Lots of riders swap cassettes all the time, to suite the type of ride.  I don't think you'll go wrong with either, after all "it's not about the bike"!  However, be aware that whatever you buy, you'll soon be looking for n+1.

If you’re going to spend that sort of cash go to a local bike shop and sit on a few bikes. Far better to get a bike that you’ve tested, fits and has been set up for you than saving 1% of system weight (you, the bike and all the stuff in pockets saddlebags etc) cassette can be swapped at point of sale anyway. 

Planet X pro carbon evo with 105 is £1000. 

Are you doing the C2W scheme?

Why do you need to spend a grand if this is your first dabble. Spend about £300 on a 2nd hand bike with mudguard mounts etc. and see if you get the use out of it. Then you'll know what you need and like and the original bike can become a winter bike. 

There's sheds up and down the Country full of good intentions and quickly depreciating road bikes!

Also the venerable Specialized Allez, I have mine built up with mudguards and it still comes in lighter than the Scott Speedster above.

Another bike to add to your list. This comes with 105 brakes and the 105 crankset. Also you should be able to see this in a bike shop.
https://www.evanscycles.com/cube-attain-sl-2019-road-bike-EV355527

https://road.cc/content/forum/262825-2019-road-mountain-bikes-now-stock-...

Bargain.

https://www.canyon.com/en-gb/road-bikes/endurance-bikes/endurace/endurac...

just a thought but if you can’t try then also have a look at direct sale options like Ribble and Canyon to get some very good deals. 

As for the gearing on the back, if you’re big, unfit or simply prefer to spin then go for the 11-32.  Especially if you’re in a hilly area. 

Honestly I would put weight down the list of your requirements unless you weigh under 60kg.