I am taking a dedicated feed from the cold water tank in my loft and then
teeing into the hot water supply. Is this best practice?

The main issue is one of sucking air through the hot water cylinder vent
pipe. It is standard practice to have a separate tapping in the cylinder for
the shower called a flange. There are several types.

However, you can sometimes get away with using the standard hot water
outlet. If this is to work, you need to have a good flow rate from the cold
header tank into the cylinder. This is helped by having large bore pipe (at
least 22mm) from a tank sitting directly above with flowed bends rather than
elbows and full bore valves. At the other extreme, it is unlikely to be
successful if you have 15mm pipe flowing halfway round the house to the
cylinder with inline isolator ball valves and elbows aplenty.

It is easy enough to see if it works first and then fit a flange if you get
air in the pump (or upgrade a seriously deficient but easily accessed
cylinder cold tank supply).

Christian.