As I see it, the advantages of sticking with a heat only boiler would be.
1. You won't have to get rid of your existing hot water cylinder and feed tanks, and possibly controls, which will simplify installation, and require less 'making good'.
2. Heat only boilers have a lot of components externally mounted. Diverter valves, hot water thermostat, programmer etc., these can be easily replaced by anyone with parts obtainable from plumbers merchants or DIY sheds. On the other hand, combi's generally have their equivalent components located inside, the components are often specific to the manufacturer and may require the attention of a gas safe registered engineer to replace them.
3 Combi’s can be slow to get hot water to the taps if the boiler is starting from cold. This is because the boiler has to heat itself first before it can heat any water.
4. To have the capacity to heat cold mains water instantaneously from scratch, Combi boilers often require the installation of a larger gas supply pipe from the meter. (This can often be seen routed around the outside of houses-Yuk!)
5. Retaining your hot water cylinder means, you can have an immersion heater to provide hot water for when the boiler fails, and a cupboard you can use to 'air' clothes, or make beer or yogurt.
6. Generally the cold main supplying a combi comes from a single 15mm pipe. That same pipe also feeds the cold taps, WC’s, washing machines, dishwashers, etc., If any of these are used at the same time as the hot water, the hot water flow / pressure will be reduced accordingly. With your existing hot water cylinder the tanks provide a stored supply of water, and are unaffected by the use of the cold water elsewhere.
...and the advantages of a combi:
1. Can provide mains pressure hot water with a good flowrate, provided no other demand is present (see 6 above)
2. Frees up space where your cylinder and tanks used to be.
3. There can be a small energy saving in the summer because of lost heat escaping from a hot water cylinder. In the winter any escaping heat is not wasted as it helps heat the property. However, this benefit can be lost if the combi has a facility that keeps it warm permanently in case you need any hot water.
4. Simpler electrical installation. But only where wiring doesn't already exist.