I'm afraid the only answer is extensive personal testing and combining that with reading up on geometry and getting a feel for the various contributing factors. There are also perceptions, personal and external, that may need to be overcome.
You could make yourself a list of potential bikes and then turn that into a spreadsheet of geometries coupled with componentry. Then try to ride each bike, with as wide a variety of setups as possible. Try your 'perfect' size in each frame and also try the size up and the size down. Ride them with a range of tyres and tyre pressures at each tyre size they can accomodate. Make notes on every variation re your ride impressions, try them with racks and guards(if they can take them) and without, with luggage and without. Look closely at any variables between frame geometries and try to isolate them for further testing. Head and seat angles, forks offsets, chainstay lengths, BB drops, stiffness and tubing diameters. Then you need to throw frame materials into the mix, try titanium, carbon, alloy, steel, mixes. Oh yeah, try to factor in a wide array of variables around bars, stems, seatposts, saddles, pedals, grips, tape, hoods, levers and gloves.
If you're reasonably thorough, but not overly anal, you should be able to make a fair start with a £100k budget and 3 years full time.
Alternatively
you could assume that the manufacturers have done this sort of thing and just try some popular models in various categories.
On a more serious level is this a case of you want to be as quick as you can be but you're not finding the full on race bikes as comfortable as you'd like? So you're trying to get comfort without sacrificing performance?
