You can do it yourself if you’re prepared to deal with all the admin. You need to get in touch with banks, building societies, pension providers etc where any of his assets were held and ask them for details of the amounts. They will probably ask for a copy of the death certificate and maybe proof that you’re the nominated executor (a copy of the will showing you have been appointed maybe).
Then you need to complete an inventory with all of the items he owned and values. You’ll need a valuation for the properties but you can probably get away with a rough estimate for things like fur