As far as I know, you definitely need a Carnet in Iran. We used it a few months ago.
If for some reason things go wrong and you have to leave the bike in Iran, you may to import it, get a stamp in your Carnet and then the deposit back in the UK, just in case. We had to do it with our car, a Renault 5 (my girlfriend said plainly NO to Pakistan right after the floods). Here's the post.
In our case, it took as TWO months to get the Iranian visa -we applied for 60 days-, although I had already gotten it two years before. Intrincate but wonderful Iran, worth any effort.
We met a couple who was shipping a VW T3 van from Bandar Abbas to India and it cost them a lot (they argued there is little traffic). Don´t know for a bike, but I wonder if it wouldn't be worth shipping it by plane: avoid extreme heat, distance, save time and even heavy Indian bureaucracy may be also easier (and that counts a lot in India).
Anyway, I still regret having missed Pakistan... (probably she doesn´t  ). Maybe you are still on time.
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