Your current Giant has pretty much the same quality components as the
Ribble. The latter is lighter, but that's about it; if you don't feel the difference between riding with full water bottles and empty water bottles - you won't feel it between these two bikes. The wheels on both can only be described as 'basic' - just there to keep the bike upright.
Being in your shoes, if I had ~£1.5k to spend, I'd buy a mid-range wheelset (
Wiggle Prime are good value, ~£600), spend the rest on a much lighter frame (good quality Chinese, like Trifox, ~£600) and swap over the remaining components from the Giant - if you can't do it yourself you can take it to a bike shop and they'll do the job, and still be within budget. Anything left over - I'd get a nicer saddle or handlebars for improved comfort, or better quality tyres and tubes combo. You'll get much, much,
much more for your money and the end result will be a light and slippery bike that's proper fast, and better build quality than anything that Giant or Ribble did in the past two decades.