I see no reason why A2A systems shouldn't be the standard in the UK, at least for new houses, other than the government won't give grants for their installation. OP if you go this route then you could just wait until there are a few more dedicated heat pump hot water tanks and just always live with an A2A heating system. 

I don't know what to suggest with regards to the best controller, but how about having your boiler thermostat set a bit below whatever you like your house to be so that it only comes on when your heat pump isn't providing enough? Eg set it to 19C if you like your house at 20C so it'll only come on when it's cooler. If you have schedules then line them up. 

You can do all sorts with refrigerant pipe runs including going round the side or through rooms to get to other rooms. It's certainly a bit less efficient to take them outside, but they can still work whilst being fairly long. Perhaps it's best to have a look where you'd put them yourself and measure the runs. Depending on various factors you might need multiple outside units though. I considered A2A for my house but nothing at all about it is even slightly open plan so I didn't think it a good fit without really piling up the indoor units. The alternative is a ducted system but I couldn't be bothered with retrofitting that. 

Holes in walls are patched normally by knocking out any affected bricks and replacing them with new bricks. You could hire a bricky to do this or do it yourself if you're brave enough. Even if you do hire someone to do it, it might be a good idea to find the replacement bricks yourself so you know you're happy with them. There'll be a brick library near you, so check it out and make sure you measure up correctly so you don't put a metric brick in an imperial hole.

Having said all that I don't actually think there's a problem with A2W installations if you're diligent about your installer. 

Some of the A2W ASHPs also can cool in summer. Most can, they are all reversible circuit heat pumps. But for MCS and 'renewable' restrictions in the UK the cooling controls are disabled and the OEM literature makes no reference to this feature. The upstairs of my house gets rather hot in summer, and the better you insulate the property to provide for efficient ASHP heating, the worse this gets. I could quite easily connect up a 28mm chilled water cooling loop to my ASHP LLH, routed up in the attic, with shunt pump, to provide chilled water in summer to 3 or 4 wall mount fan coil units in the bedrooms. It's something I might do once the OEM warranty has expired. The controls & parameters would need a little tweaking, including a flow switch, etc., but it's all do-able. I would have to interlock the existing central heating secondary pump and hot water controls, etc., but in summer the iBoost and Solar PV will take care of my hot water needs.

Posted by: @allyfish

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Some of the A2W ASHPs also can cool in summer. Most can, they are all reversible circuit heat pumps. But for MCS and 'renewable' restrictions in the UK the cooling controls are disabled and the OEM literature makes no reference to this feature. The upstairs of my house gets rather hot in summer, and the better you insulate the property to provide for efficient ASHP heating, the worse this gets. I could quite easily connect up a 28mm chilled water cooling loop to my ASHP LLH, routed up in the attic, with shunt pump, to provide chilled water in summer to 3 or 4 wall mount fan coil units in the bedrooms. It's something I might do once the OEM warranty has expired. The controls & parameters would need a little tweaking, including a flow switch, etc., but it's all do-able. I would have to interlock the existing central heating secondary pump and hot water controls, etc., but in summer the iBoost and Solar PV will take care of my hot water needs.

I have heard you have to be careful with cooling as a lot of condensation can be produced on the cold pipework.  Also, doesn't your ASHP cease to be permitted development if it's used for cooling. hence specific planning permission is required?

I suppose another incentive for A2A is in a move away from the central heating idea of heating all rooms that could conceivably be used during the day which may result in some energy savings.  A2A allegedly has a much faster warmup time which would permit rooms to be heated on demand.  I used it in that way with a rented cottage in Italy, just turned it on in the room I wanted to use 30 mins ahead.  Whether it's still workable in UK remains to be seen.

@ronin92 

I think it's something that should be encouraged here as current A2W ASHPs don't work well heating single rooms.  Not everybody wants their whole house heated.  Current planning regs aren't very helpful though.