Planet-X Pro Carbon or RT-58 for 'fitness riding' bike, - or something else? all advice appreciated, TIA!

Should first say "Hello", since this is my first post here.

My question regards Planet-X road bikes, and whether it is worth spending the extra £200 to get the RT-58, over the Pro Carbon.

A bit about my 'journey'. I used to cycle a bit back in my early 40's (55 now), but was mainly a runner back then. Knees gave up, and all exercise went out the window back in 2002, and soon saw weight go from 175lb to 255lb. ( I'm 6'1", so 175lb is middle of 'healthy', 255lb is verging on morbidly obese)

In January of this year, I decided to do something about it. Cutting back on excessive eating and walking briskly, saw weight start to come off and resting pulse start to come down (BP had always been OK). I've used a Dawes Horizon off and on for local commuting, but began taking this out for short (12-13 mile) rides, just for the fun of it. Over the course of these last 5 months, I've dropped 70lbs, I've now ridden up to 60 miles in a day (with 4,100 of ascent), on the Dawes tourer, and I think it is time to treat myself to a 'nice' road bike.

Don't really want to spend much more than £1000. (Youngest daughter still has 3 years of Uni left in the USA, so not too much 'disposable' income)

So there's the question. I want a bike I can ride a 100 miles in a day on. I may even think of entering a Sportive, but it is primarily about getting a nice bike to simply go out there and enjoy being a healthy weight again. I don't have significant back issues, but can get twinges.

The question is: "is the RT-58 worth me spending an extra £200 on over the Pro Carbon, for my predicted purposes?"

Really appreciate any advice that can be given

Regards

Kenneth Ross