The two highlighted kinda conflict, you wouldn't find many places doing a complete custom build with cheap enough quality parts to make it under £1100. That might sound awkward but just hear me out...!
You're at the stage you want something 'better' or just more specific than you had before, equally your experience is limited to what you've already ridden and know, no matter how small that experience is. You could go through 'shopping' for parts by reading reviews for this that and the other online to then put together into a build... BUT you really need real world experience of said parts together to determine suitability/comfort/compatibility. If you go by reviews it's really is pot luck if you end up with components perfect for you/your build as the majority of reviews are quite specific to the reviewer and bike they are using...
It's a bit catch 22 - you need a way to try some of the parts out in an unbiased and balanced way to make your own decisions. It can mean buying 11 saddles and trying them all before selling 10 of them. Or it could mean getting your sit bones measured as part of a bike fit to determine more specifically what width of saddle suits you, then deciding if you want it firm/solid and the miriad of other 'features', then using this information to try and narrow your search (buying 3 and selling 2).
Really it's a minefield without having friends bikes to try or loan bikes from your local bike shop to try. For example, I rode with the 'Giant Contact' saddle on my Defy 1 (2015) for probably 12 months before changing it to a Charge Spoon, less than 2 months changing it to a Fizik Arione. I found it loads better than the Giant and Spoon but as I gained fitness and rode much more I started to struggle with saddle sores. I changed to various different more expensive shorts and found them better, increased mileage again and suffered again. When I bought a new bike (Specialized Diverge) it came with a totally different saddle than the Arione - a wider nose, a central relief channel and more gel, it's been so comfortable I actually bought another to put on my Giant! I still suffer with sores occasionally but nothing like before. Equally my experience of saddles is only 5 different saddles. There's probably a better saddle out there for me, but finding it will mostly be down to luck!
