Something like this: Carrera Virtuoso Road Bike 2015 ?

Pretty basic bike, not terribly heavy .... low-end but serviceable parts. But ...

I wouldn't put much money into it. pardon if I get my calculations wrong, because I am just guessing, but ... wheels £200, tires £30, drivetrain £250 or so for 105 .... why not ride the snot out of it and love it and save up for a better bike a year or three down the road?

You could pick off cheaper used parts on EBay and upgrade it over time for a little less, but essentially you would spend two or three times what the bike cost to upgrade it as you suggest .... and you'd have one bike and a lot of parts. Why not spend the same cash and have two bikes?

Unless you are fit and ready to start competing today, I'd say use this as a training bike and when you are ready to actually pay to compete, pay for a bike which will help you to compete?

And you could ride this on sportives exactly as it is. other than what I would assume are a few heavy/cheap parts (saddle, seat post, wheels) there is nothing on that bike Needing to be upgraded.

And just as it stands at what 12 kg, it is fine for any ride. You will have to work a little harder, particularly on the hills, but you are training anyway, right?

Also, if you just bought the bike and haven't done a lot of riding, your first concern should be learning to ride, and then, the more advanced course, enjoying riding a bike, learn basic maintenance and adjustment (if you plan to swap parts you will need that anyway) and find out about group rides in your area, if you are so inclined ... or do what i do: get on a mapping website, look at your area, look at which roads wiggle a lot (usually less traffic) and connect to form loops of various lengths, and map out local rides.

I have loops from about 10 miles to about 70 heading out in all directions from my home, and I can string them together or cut them short (some of them) so I don't have to ride the same roads every day.

Ride the bike, learn what you can do with it ... enjoy it. When you know exactly how serious you are about riding and competing, decide then what bike will help you reach your goals.