Panel heaters

[STI]

Why do "fixed" panel heaters need to go on a fused spur and yet I can go to my local DIY shed buy a (or several) 2kw panel heaters that can be plugged in.

The plug effectively acts as a fused spur.
Whether plugged in or "hardwired there is a possibility of overload to the unwary.

So why is it required by the manufacturer and the regs?

 

[winston1]

They don't need to go on a fused spur. Fused Spurs are not available or compliant in most other countries anyway.

You don't have to follow manufacturers instructions, especially when they are wrong. You have to do it safely.

 

[John D v2.0]

If something is fixed rather than portable that means it's probably going to be used more.
Plug connections are not perfect and can get hot under high load. I'm sure we've all seen sockets that have been damaged by long term use of fan heaters.
So best practice would be to minimise the risk by hard wiring.
Of course it's not possible to stop people doing everything that causes risk - you could plug 4no 3kw heaters into each ring, doubled up on long coiled extension leads, but clearly that's ridiculous. So the rules are all about finding a level of risk.