How to calculate how much current is available from Buck regulated Solar Panel
The first part of this answer is to show that the OP misrepresented the numbers from his data by suggesting that the daily energy average was 470 kJ/m2: -

...suggests that the average (incl. night) for the whole year was
470KJ/m2


470 kJ is energy for the whole year per square metre.

So divide that by 365 to get energy for 1 day = 1288 joules.

Then divide that by 24 to get energy per hour = 53.7 joules.

Then divide that by 3600 to get the equivalent average energy per second = 0.015 joules.


This is a power of 15 mW per square metre.
I think you might be somewhat overestimating what power you will receive. Maybe your numbers are wrong or you are misinterpreting them: -

You're right, I've clarified that the 470KJ/m2 was the average per
hour. – Adam Leyshon


Here are some solar energy numbers from NASA for the Midlands in the UK (about the same for Wales I would imagine): -

We can see that a reasonable average daily energy per square metre is around 2.5 kWh for the UK. That's an average daily power of 104 watts per square metre.

Image taken from this site and 104 watts per square metre ties in nicely as an average.
If we factor in the solar panel's energy efficiency (maybe 20%), it becomes about 21 watts per square metre.
Given that the solar panel linked to in the question has dimensions of 185 mm x 250 mm, that's an area of 0.04625 square metres so, the viable average power that could be liberated is 0.963 watts.
Given also the efficiency of the converter is 80% then you can only really liberate about 0.77 watts
If you are wanting 12 volts, then the average current available will be about 64 mA.