When speaking to other British people, I would always naturally pronounce 0 as 'oh'.
In a more mixed environment (foreign speakers, speakers of other dialects of English), I would say zero because it is easier to understand and more widely understood. Oh can be misheard but zero cannot. Some speakers of English only recognise 'oh' as a letter.
As this is the emergency number of a country where English is not a native language, I'd say one one zero.