How can I prioritize a SPARQL query to query for one thing, before querying another?

Question

The query below simply goes to Wikipedia and grabs famous people born with the date given. I want the query to first prioritize famous people born in the USA first, then the UK, and then everyone else. This will eventually be for a website whose traffic demographics will be from those countries mainly.

USA = wd:Q6156 Great Britain = wd:Q1833

SELECT ?person ?personLabel ?image ?birthdate ?description WHERE {
  ?person wdt:P31 wd:Q5;
          wdt:P569 ?birthdate.
  OPTIONAL { ?person wdt:P18 ?image. }
  OPTIONAL { ?person schema:description ?description. FILTER(LANG(?description) = "en") }
  FILTER(?birthdate = "${formattedDate}"^^xsd:dateTime)
  SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }
}
LIMIT 30

I tried this

SELECT ?person ?personLabel ?image ?birthdate ?description WHERE {
  ?person wdt:P31 wd:Q5;
          wdt:P569 ?birthdate;
          wdt:P27 wd:Q30.
  OPTIONAL { ?person wdt:P18 ?image. }
  OPTIONAL { ?person schema:description ?description. FILTER(LANG(?description) = "en") }
  FILTER(?birthdate = "${formattedDate}"^^xsd:dateTime)
  SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }

  FILTER (?birthPlace = wd:Q6156) UNION
  FILTER (?birthPlace = wd:Q1833) UNION
  ?person
}
LIMIT 30

but the query is somehow malformed. Not sure what I'm doing wrong.