As far as I know there are no wall hung oil combi boilers. The shear size and weight would require some serious mounting to a solid wall. If there is such available then I apologise but i've not come across one, ever.
Yes installing an oil combi boiler will allow you to get rid of the HW cylinder and the tank in the loft. Yes you could fit a thermostatic shower but bare in mind combis are usually sized for HW flow rate performance, having said that an oversized combi for HW performance is not good on an oil combi. They do not fully modulate like their gas cousins currently (although the technology is heading that way), so installing a 35KW combi for good HW performance is not good for condensing if your CH load is only 15KW for example.
When designing a heating system many things have to be considered, one of which is the flow rates required to the emitters, yes microbore off a manifold system would work as each emitter (radiator) will be fed of its on microbore loop. Providing the maths is done then it will work if it adds up but you certainly wouldn't run multiple radiators of a single microbore loop. The other good thing about microbore manifold systems is they can need very little or even no effort to balance as each sub circuit is kept to an almost identical pressure loss. The bad point however is as I mentioned above, they can clog up more readily. Providing the new system is flushed, with the right inhibitor and concentration and suitable dirt and air separation then the chances of that happening is really quite low. Now you mentioned an ASHP? I can tell you now if that was considered at a later date after you had an oil system installed on microbore then virtually all of the system will need to be redone as the pipe sizes will be inadequate for the higher flow rates and lower delta t of an ASHP, many of my associates are now recommending future proofing for such systems.
With regards to heat loss calculations it needs to be done properly, no cutting corners. There are several ways to go about it but perhaps the best and most complex route is a website or app called Heat Engineer. This is created and run by an associate of mine and is very, very good at giving all the right information, the only problem is you will need a lot of information about fabric of your building to do the calculations. Failing that your engineer will be able to do it all for you when designing and quoting. Another thing I will point out is if you were to upgrade to an ASHP at a later date then the radiators will need to be larger due to the then new low temperature system and the existing being undersized. I suggest you think hard about which energy source you take on. I personally have an ASHP for my flat. In the warmer months its electricity use is very cheap, however in the colder months my electricity a week is doubled at least but thats how these units work and is part and parcel.
Electricity can be one of the best efficiency wise for energy transfer, although its also the most expensive.
Edit: ASHP's would be useless in an older property with poor envolope insulation. If this is a route you're going to research and possibly pursue then you will need to upgrade the insulation of your property to a better level.
