Asrock Fatal1ty Z370 Gaming K6
Fractal Design Define S "Silent" Mid Tower
Corsair Vengeance LPX 2x8
Budget £50-100
Motherboard has 90.8mm center CPU to near side of x16 PCIe and 70.8mm to near side of x1 PCIe but most peeps don't use it anyway. RAM socket is 50.8mm center CPU to near side of nearest RAM socket so good clearance in all directions.
Vengeance LPX is only 31mm tall.
Define S has 180mm CPU clearance so no problems there either.
Your up to £100 allows for all air cooled CPU coolers and I do not recommend CLCs because the cost more, generally make more noise, don't last as long and if something goes wrong it's almost always the pump so no CPU cooling at all until new cooler is installed .. so no computer to use plus cost of all new cooler.
Starting at the highest priced be quiet! Dark Rock Pro 4 has both looks and cooling at £80, Dark Rock Pro 3 is almost as good at £75, NH-D15S is good at £75, Cryorig R1 Universal has both looks and cooling at £73, NH-D14 is still one of the best at £71 but does not have PWM fans, NH-U14S is good at £63, PH-TC14PE is quite good at £60. That is most of the top tier that OcUK stock. We can add Thermalright TRUE Spirit 140, Le Grand Macho RT and Silver Arrow IB-E Extreme available elsewhere. I wold probably go for Dark Rock Pro 3 or 4, R1 Ultimate, PH-TC14PE or NH-U14S
Honestly at this point you can choose almost any cooler you want, so what ones do you like?
I will add that Dynamic GP14 case fans included with your case are almost worthless so a couple new front intakes would be a good idea. What GPU are you using? I've found 2x-3x good intake fans speed controlled to cycle with component temps with all PCIe back slot covers removed to increase rear vent area around GPU almost always give good case airflow with low noise, even at full load. One of my favorite is PH-F140MP for about £14. but most are out of stock right now. Next up is something like Fractal Design Venturi HP-14 PWM (PWM are my preference) at about £18 and best are be quiet! Silent Wings 3 140mm PWM High Speed at £2. The other extreme is Arctic F14 PWM or F14 PWM PST (has female socket for daisy-chaining fans) for £7. While not as good are others are not bad and very low priced.
Edit: Alphacool customers have some real horror stories trying to resolve problems with them. I think I would avoid them.