What's the difference between S W or Y plan?
Out of the three what would you recommend and why?
Would the above type of system be better than a combi?
S-plan has two 2-port motorised valves, one for CH, one for HW. When CH or HW is called, by the relevant stat, the valve opens. To avoid risk of pump and boiler being dead-headed due to a valve failing closed, the pump and boiler are switched by a microswitch in each valve, switch closed when valve fully open, rather than direct from the stats. This makes the wiring a bit more complicated. If both CH and HW are called, both circuits get flow.
W and Y-plan both use a 3-port diverter valve. W-plan is either CH or HW, not both, and HW has preference. Y-plan has a mid-position valve so both circuits at once can get flow. Body of the valve is the same, just different actuators. Because of the valve design it's impossible for both ports to be closed at once, so the pump and boiler are powered directly via the stats. I've always had W-plan and never found HW preference a problem, even with 5 people in the house and plenty of baths happening. W-plan valve is simpler and maybe more reliable.
Also if you ever go in for weather compensation, where in effect the boiler control-stat setting is reduced when outside temperature is higher, to improve economy (more condensation) this is only in CH mode. As the reduced control-stat setting might be below the cylinder stat setting, it clearly wouldn't do the job, so it reverts to normal control-stat setting in HW mode, and the system works in either/or mode, making a mid-position valve superfluous.
Personally I prefer a heat-only boiler to a combi, for one thing HW cylinder with immersion means you still have HW if there's a problem with the boiler. And with the various items being separate it could be easier to diagnose and fix any problem. Installing a combi likely to mean more upheaval than my suggestion.