Half sovereigns should weigh 3.99grams and are 22ct gold.
What you have here is what is called a shield back half sovereign.
They are easy to sell as gold but some do hold a premium but to be fair this looks a bit worn.
On the reverse at the bottom should be a mint mark
S for Sydney
M for Melbourne
If no mint mark could be British
Thanks mate. I was hoping I had found some prized ancient artifact (: