Replacing unvented hot water immersion system

Electric combi boilers are okay, you will still require a hot water cylinder and therefore it would be cheaper initial cost just to put electric immersion heaters within the cylinder although this would be more expensive to run. Also with an electric combi boiler you can run a wet heating system (thats the main point). This would be far more efficient than having electric panel heaters etc. If you are only looking at hot water (and not heating) as your question suggests then I probably wouldn't look at an electric combi boiler.

If its just for hot water to supply sinks, a 15litre unvented cylinder is a good solution, if there is a bath , a larger tank is required. The cylinder ideally would be sited fairly close to one of the sinks , plumb into the waste and hot/cold water, requires 13amp electric.