I'd go with a mod-con gas boiler. These can be purchased with mid-90% efficiency. The nice thing is that they can adjust their output to match the load. Having the heat on constantly, but at a low level is the most comfortable, and when the boiler runs at a low level, it is more efficient than it is when at full output. Then, get an indirect WH, which uses the same, high-efficient boiler for the heat, but gives you fast recovery and no flow restrictions if you want to fill a tub, or have multiple taps running at the same time as you'd get with the in-boiler coil.