Although I agree with your sentiments I feel that I have to defend ONS and Statisticians in general here.
When we develop forms or databases we try to think of every eventuality that could possibly occur, but its not possible. For example, we have a database which accepts any postcode in the UK, has an option for missing postcode (homeless) or out of country home. We hadn’t considered Womens refuges, which often follow a completely different system.
Another example, we often use 999 or similiar to represent something being missing off the form. We hadn’t considered the fact people being born on the 9th September 2009 would be coded as “missing DOB” by mistake as a result of this.
Often things look a bit “funny” and I dig out the old data forms. Often I discover a perfectly legitmate explanation, which I simply can’t code, usually because the numbers of it occuring are so low it isn’t sensible too. However, I read the original forms and smile and their odd circumstance has been noted by me.
In the future that unusual circumstances may be mentioned in the set of a new database (as was the case with the DOB issue). ONS may not record what you want now, but that doesnt mean at some point they won’t change it.