Nothing wrong with Hep20, or microbore, use the barrier pipe, Hep20 doesn't come any smaller than 10mm anyway, just make sure you use the proper cutter, make sure your pipes are large enough to take the amount of btu you are trying to get through them, I run large pipes up to a convenient point then reduce down to microbore about last three metres, only use 10mm pipe for rads less than 10,000btu, any bigger use 15mm, you wont get any sludge problems with a sealed system with a modern boiler, especially if you fit it as per flushing and inhibitor specification.
You can always use copper where pipe visible and chage to Hep20 wher hidden