I am thinking of entering in the day too,see the email from the race organiser below. It seems to be the only race in the South East which is accepting entries on the day.
Yes there will be entries on the day (£8 for members of UK Athletics-attached clubs, £10 for unattached runners). There is some limited parking on local roads, but Finsbury Park station and Manor House station are both less than 10 minutes walk from the start.