First of all, do a heatloss calculation. That'll tell you what your losses actually are. I'm going to assume 10kW is approximately right for your property - it sounds about right for a large-ish regular UK house with moderate levels of insulation.
That figure is a 'worst case' calculation - for the average coldest winter day at your location. On days that aren't the coldest, the heating load will be less and your heat source will be expected to modulate down. Any appliance only has a certain ability to modulate - eg down to 25% or 50% of the rated capacity. Below that it'll have to cycle - turn itself on and off to maintain the required output. This is less efficient than running constantly at a lower load. Cycling is not good news for heatpumps, and not ideal for boilers either - but many boilers do it because they are oversized.
10kW is a fairly small amount of heat. That's quite a small gas boiler. A quick look on the Worcester Bosch site had the smallest one it offered me being 12kW. So even on the coldest day it'll already be in modulating territory. Of course, having extra power will help with warming the house faster and faster water heating, so it's not all wasted.
So it seems to me that 10kW is an average air2water heatpump install in a pretty average house and, assuming installed correctly (sadly still a big issue), that will provide all your heating and hot water needs. It would seem that a 5kW ASHP plus a minimum-sized 12kW gas boiler would overcomplicate things and cost a lot more to install (once you're installing a heatpump the size of the unit doesn't really affect costs very much).
For hot water, let's say you have a 250 litre cylinder. To heat it from cold, input temp of 10C to 45C takes 36.75MJ or 10.2kWh. With a 10kW ASHP at optimum output it'll do it in an hour and take 2.5kWh of electrical energy. On colder days it might take a bit longer, if you want it hotter a bit longer and more energy again. Why would you need an additional gas boiler for this? It'll heat it quicker, but unless you have a lot of people bathing in quick succession the ASHP can recharge the cylinder just fine.
Data point: I have a 13kW Aerona3, house heatloss somewhere between 5 and 8kW. Last week it was -5.5C outside when I woke up and it coped with absolutely no problem. There is scope for an electric backup heater when it gets really cold (-20C kind of level) but there is not much need to wire those in the UK. Most of the year the weather is not sub-zero and you reap the rewards in efficiency on those days, which more than compensates for a few days at lower efficiency. 200 litre cylinder gets heated from cold in about an hour, but one tank is enough to have a couple of back to back showers.
The only issue I can see is the microbore: heatpumps need larger radiators to run at lower temperatures for best efficiency, and larger radiators need more water flow to them. If it's impractical to replace the microbore with larger pipes then that could be a problem for a heatpump install. Flexible PEX or polybutylene pipework might allow replacement in awkward spaces.
Finally, there is the question of whether you trust your electricity supply, and what happens if it goes out? Having a backup plan is good. For example, in a rural area where outages are more common, you might have a wood burner. In an urban area you could keep a gas bottle heater. Worth thinking about, but I wouldn't let your one-week-per-decade outage determine your heating choices for the other 519 weeks.