Postgres point() index not being used

Question

\d aggregate

              Materialized view "public.aggregate"
          Column      |            Type             | Modifiers 
    ------------------+-----------------------------+-----------
     id               | integer                     | 
     searchable       | text                        | 
     name             | character varying(255)      | 
     source_type      | character varying(255)      | 
     source_id        | integer                     | 
     latitude         | double precision            | 
     longitude        | double precision            | 
     created_at       | timestamp without time zone | 
     updated_at       | timestamp without time zone | 

Indexes:
    "aggregate_lat_lng_point" gist (point(latitude, longitude))
    "searchable_tsvector" gin (to_tsvector('english'::regconfig, COALESCE(searchable, ''::text)))

Query:

EXPLAIN ANALYZE SELECT name FROM aggregate 
WHERE point(53.574753, -2.1) <@> point(latitude, longitude) < 100;

Result:

 Seq Scan on aggregate_mv  (cost=0.00..23.01 rows=172 width=516) (actual time=0.019..0.522 rows=458 loops=1)
   Filter: (('(53.574753,-2.1)'::point <@> point(latitude, longitude)) < 100::double precision)
   Rows Removed by Filter: 320
 Total runtime: 0.579 ms
(4 rows)

As you can see, the index is not being used. The table contains 525 rows, and the above query returns 195 rows. The table is a materialized view, but that shouldn't make any difference, I have other indexes which work fine. Any ideas as to why my index is not being used in the above query?