Hope this helps you:
with cte as(
SELECT id, string_agg('"' || tag || '":' || count, ',') AS tag
FROM Table1
GROUP BY id
)
SELECT c.id, ('[' || c.tag || ']') FROM cte AS c
Answer is of the form :
uid | json_agg
-----+----------------------------
15 | ["latergram":1, "wales":1]