10mm is fine. It is very usual to bring 10mm to rads from a 15mm feed which
in turn comes from a 22mm flow and return. 10mm is also very easy to work
with on a DIY basis. All you need is a hand held bender and you're away.
It's easier to install than 15mm in many places. Fittings are more expensive
though. As for the TRV.... he could have used a 15-10mm resucer and used
your TRV so make sure he don't try and charge you for his one. A swap sounds
fair though - he may not have had a reducer so fair enough. These days 10mm
is used for many other things such as tap feeds. Brand new houses have 10mm
tap feeds coming from a 15mm pipe. I used 10mm a lot in my new build. You
simply don't need the flow of 15mm unless it's the likes of a bath or
shower.