Some embassies ask for a monthly income, some do not. They just want you to have 500,000 THB and it's always good to show you have additional funds too.
As Kyle has said it's really best just to ask the embassy in the UK - things change so fast. All the best with your DTV and time in Thailand 🙂 But from the best of my ability and what others have done the link above is fairly accurate.
"The DTV visa allows the principal holder to bring in dependents. There is no limit to the number of family members, but they must be either a spouse or dependent children under 20 years old, and each dependent must apply and pay the visa fee separately."