Surely the flow is restricted by the smalled pipe?

This is where you misunderstand. The smaller pipework doesn't limit the flow
rate, it just provides resistance. A little bit of 15mm won't harm. It is
long runs of 15mm that cause the trouble.

Hence 10m of 22mm pipework followed by 1m of 15mm pipework is almost as good
as 11m of 22mm pipework and is much, much better than 11m of 15mm pipework.
If you see what I mean.

Think of it as a road network, with 22mm being a nice motorway and 15mm
being a twisty B-road, but without any queues. You don't mind a bit of
twisty B-road at the end, provided you cover most of the distance on the
motorway.

I have no idea if their design is sufficient, though. A large cooker and
combi boiler (i.e. at least 24kW, probably more) can be quite limiting on
22mm pipework. My installation has 28mm until the boiler/cooker tee and then
continue in 22mm until the appliances, although the runs for both are quite
long, particularly after the tee.

Christian.