"cust. no." should be easy to understand.

Agree with #1

Are you short of space?

C#

#3: C#

I wouldn't understand that - unless it was absolutely obvious from the context / explained.

... 'customer #' seems to be very common to me ... always depends on how much space you have and if you really need to shorten it ...

Has the hash sign to mean "number" caught on in the UK?

This blog from 2010 suggests not:

The use of # to mean 'number' is increasingly understood in the UK, but not often used in that way.http://separatedbyacommonlanguage.blogspot.co...

#6: although understood by many, it is by no means standard in the UK.

As always, I agree with CM2DD.

If you actually want native speakers to understand you....use cust. no.

All these symbols, although some people might understand, they're still gonna look strange.