Sorry, just looked at my daughter's British Birth Certificates, short and long versions.

No box for middle name.

Just says:

Name and Surname Xxxxx Yyyyy ZZZZZZZ

With Xxxxx and Yyyyyy being the Names and ZZZZZZZZ Surname written in capitals.

The formal distinction you were imagining just does not exist.

What you have described for British names fits equally for Spanish ones, there are plenty of Spaniards who use one and not the other. The ratio of how many people who use their first vs their second or subsequent names might change a bit according to country, and even according to class and region, but it honestly isn't as cut and dry as you are making it out to be. Saying there is no concept of middle names in Spain really means that there is no concept that fits exactly with the one you have, which is totally different to mine (at least I imagine it is, I still don't really understand what it is you are describing)

Worth remembering that back in 1911, 63% of the UK population had only one given name.
http://blogs.ancestry.co.uk/cm/tradi...-middle-names/