Iran is doable w/o the CdP. I got iranian company to backup (they guaranteed) my case that I'll not sell my GS in Iran. But in my case I had to return with the bike from the same border post I entered the country.
Actually bikes more than 250cc are still banned in the contry, but this law seemed not to apply to foreigners (I was on my R1100GS).
With CdP it's easier crossing of the border, but buying CdP from UK costs you a fortune of deposit money depending how expensive your bike is. I'd recommend issuing CdP via German ADAC, 3000EURos deposit (this money you get back later when you return with the bike) per vehicles with market price up to 15,000EURos. 220EUR issuing fee only that you don't get back.
Compared to UK, Canada etc Carnets, it's a bargain. Check if they issue CdP for UK citizens tho.
PS. You live in Oman then?
Good roads, Margus