Since you've got an Evans near, buy this - it's a bargain and perfect* for your planned tour:https://www.evanscycles.com/pinnacle-dacite-1-2019-touring-bike-EV318241
When you you start training, you won't get any significant benefit from a lighter more race oriented bike. A touring bike will enable you to get plenty of good quality training miles in, and do it in relative comfort. That will help you get faster much more effectively (i.e. by getting fitter and probably losing weight in the process) than buying a bike that is more race oriented. It's a relatively heavy bike to race, but that is inconsequential set against your own weight. Moreover, if a race has a flat profile with few if any hills, the weight of the bike (and your own weight) is far less important.
As you get fitter and closer to the actual race, you can experiment with things that will increase speed, e.g. buy and fit some tri-bars to the handlebars, and maybe experiment with lowering the handlebars for a more aero position, fit some narrower/faster tyres (the internal rim width is 21mm, so I think you would be able to fit a 28mm race tyre), and maybe switch the rear cassette for a closer ratio one if you start to find that the jumps in the gears are too large when you are going absolutely flat out. Finally you can remove the mudguards and rack for the race to get that extra bit of speed.
* Edit - I've just realised it's got 32 spoke wheels instead of 36, which is less than ideal for your weight. However, one of my reasons for suggesting the bike is that since Evans are local to you, you should be able to go back to them and get the wheels re-trued if your weight results in them going out of true. As you ride it (just touring, not even training), you should find that you get fitter and lose weight without consciously trying to do so. So even if you have problems with the wheels going out of true early on, that should stop happening after you've lost a bit of weight.