[qubex]

When I was taught English it was most definitely frowned upon and disparaged as “at best an Americanism”. It is grating to the native British ear and has no place in formal writings. There is no situation where it cannot be avoided by rephrasing the sentence (usually, by no more than employing “will be going”, but occasionally resorting to other constructs). During the IB we were absolutely forbidden from using it and would be marked down severely.

[foldr]

I'm a native British English speaker and it sounds completely normal to me. There's certainly nothing grammatically wrong with it. It's the same structure as "going to eat" or "going to walk".

Marking you down for using an expression used by all native English speakers is bonkers.

[cat199]

https://www.youtube.com/watch?v=GyYRyhvlf48

example #1 from a clearly educated british person:

'i was going to go' (yes, no negation, but still..)

agree, this is not formal language, but quite common.