The microbore pipes might not be an immediate no-no, but some investigation would be needed as to whether they would work along with the radiators. Heat Geek have a youtube video on the issue. To keep costs down in my install, I was able to just buy a couple of very large radiators and then move the replaced ones into smaller/bedrooms, in the end only getting rid of a couple of small ones.

Five of our 14 radiators are supplied with 10mm plastic microbore, with the rest on 15mm copper. The microbore is all installed behind plasterboard walls, so would be a pain to replace and we'd probably end up with surface mounted copper instead, which would add significant cost.

Some of the installers we spoke to expressed concern and/or dire warnings about using the microbore with a heat pump, while others were confident that it would be fine (one suggested we might consider a secondary circulation pump on this part of the loop, but only if necessary).

From what I can gather, those in the know consider 10mm microbore to be OK, but not 8mm, so this might make a difference to your options.

I'm just about to book in an installation without upgrading any of the pipework, so will find out the truth in due course.  I've been running the gas heating at 50 or below for a while now and, although it can take longer to get up to temperature, once this is achieved, it seems to work fine, so I have reasonable confidence.

I have 10mm copper microbore installed behind dot & dab plasterboard for the downstairs radiators. I’ve done my heat loss survey and have concluded that I will probably have to replace most of the radiators and add a 3rd vertical radiator in the kitchen/diner. I’ve done the maths on the pipe work and will need to repipe the existing two as the microbore won’t deliver the required heat to the kitchen diner I also have a problem with a short 15mm section under an upstairs floor, but it’s manageable. For the remaining downstairs radiators, the pitch of the rad valves at either ends of the radiators and their distance from the wall will have to change. In order to get project approval from my dear wife, I have to avoid the need for redecorating (although I can’t see how this can be avoided in the kitchen/diner). I’d be interested in seeing photos of how others have addressed replacing radiators in such situations (not necessarily with microbore). It seems to pose a three-dimensional puzzle as well as risk additional obstacle to the flow. 

@technogeek, we’re going to have to address this issue with our system. Meeting Ken Bone this week was incredibly enlightening. One key takeaway from our chat was the importance of getting kW down the pipe, which boils down to velocity. Our system also uses microbore, but that's far from a deal breaker. I'll keep everyone updated on our progress, because our system—whether as a whole or in parts—is like many other retrofit systems across the UK, which means there's a lot to share and learn.

What I have learnt so far is that designing for microbore, it's crucial to ensure that sufficient water is supplied to deliver the required heat at the designated Delta T (∆). In some cases, adding an additional pump may be necessary to achieve the required flow rate. A buffer (but a well designed, balanced and installed one) might be necessary. To account for this, consider adding an extra 10% allowance to the radiator size on the relevant circuits. Additionally, using the highest flow rate fittings possible is important to minimise parasitic losses and ensure efficient system performance.

@ianmk13 I assume you've considered upgrading from K2s to K3s where possible?  This should give you at least 30% extra output for the same wall space.

to the OP, one important factor will be the size and length of the run to the manifolds from the radiators and what size the feed to manifolds are. 

@mattc All the existing (designed for T50) radiators are single panel, only around half with fins. I think the system was probably well designed when the house was built 25 years ago. Most, if not all, radiators will need replacing but we only fitted the tall column radiators last year (I blame myself as much as the plumber who spec’d them for a lack of foresight). 
I can only imagine that replacing the radiators will require some Heath-Robinson adaptations at each end.