You're new to cycling according to your user name. As others have said you can't buy speed. Personally I'd start with something much lower cost and build speed, strength and stamina. You will eventually reach a point when you realise you could go faster with a better bike. That will probably take 2-3 years.
For £4.5k you will get a beautiful bike but will it really make you go faster at this stage?
Don't forget very good wheels will bring you significant benefits. My experience is quality wheels did as much, if not more, to improve my riding as my bike which was £3.2k three years ago. I invested in the wheels 18 months ago.
There is much, much more to improving one's riding than an expensive bike.