Generally speaking, in addition to showing financial health bank account statements are used to establish or confirm ties to the country.
The fact that is there x amount in the bank is only of secondary importance.
The embassy officer is looking for:
Do you have regular expenses, and regular income? (this shows ties)
Do you have enough funds to cover the expenses of your trip, and not be a burden on the state? The amount that the officer is looking for depends on the length and purpose of your trip and/or if any part of the trip is pre-paid or being supported by someone else.
Is your financial position / lifestyle commensurate with your job? Do you have any undeclared income that you didn't show?
I have never seen credit cards being used to satisfy all the above. If your father is retired, then he can show his pension statement. If he is receiving payments from other businesses in cash, he can show receipts of the same, or a letter from a recognized, accredited accounting firm that handles his business accounts.
I fear that providing a large sum in the form of a credit card would raise flags and this is akin to funds parking (that is, borrowing large amounts of money for the status of inflating your financial situation in order to obtain a visa). If suspected of this, the application is almost guaranteed to be rejected.
I do know that credit cards have been used as secondary sources of funds; for example when asked by the officer how do you plan on supporting your self, one can answer:
I have x in GBP available to me in my bank account from my savings,
in addition I have access to x GBP available through my credit
The correct way for you to support your father, is to provide an affadavit of support; this is in a form of a letter addressed to the embassy, it should state:
- Your full name, passport information and current status in the UK.
- Your address
- Your job details (if you are employed).
- Proof of relationship with the sponsored (if applicable)
- A statement that you are willing to support the person during their stay from ________ till _________ for the purpose of _____________ .
- Any addition form of support (for example, you are paying for their ticket, the person will be staying with you - or - you have arranged for a hotel), etc.
Along with the letter a copy of your passport and visa page, or residence card. Proof of accomodation (this could be a lease for property, or proof of a hotel reservation, or copies of utility bills stating your name and the address).
Your father can then supply this to the embassy as proof of funds.