If you are happy to buy without riding it* then I would recommend checking Canyon and Rose Bikes. Canyon are cheap but very nice. They have pedigree from the Pro Tours so they can obviously make a good bike. Not ridden one but I’ve seen a few and they look very good.
Rose are similar in that they are a direct supply German company. They peg prices to the exchange rate so are cheap at the minute. Full carbon Ultegra is about £1700, Di2 is £2000 and you can get Dura ace for about £2300. Vastly cheaper than anything else I’ve seen.
105 works well, I’ve got it on my cross bike. Ultegra works very well, I have it on my road bike. I don’t think there is much in it other than weight. I would check with the bikes to make sure they are not hiding cheaper components. It is very common to stick on an Ultegra mech because people look at that but then use cheaper hubs, bars, posts, saddles etc. because people don’t notice as much.
*it seems you are if you are considering ribble. IMO provided you can work out the frame size test rides are over rated. I couldn’t have told you if my road bike would be any good having ridden it for 1hour. I’d need to race it, test on it, take it on a century ride and generally get used to it. Sometimes things are different and it takes tmie to realize if different is better or worse.