I would advice to send a query to home office (in email) and ask them if you can extend your visa before it expires. I am sure you will receive a reply saying
"you can extend anytime before it expires but it is recommended to apply within 5 weeks of expiry". You can then use this email with your application and apply.
I have sent enquiry to HO and got the same reply. I don't think you will then need any justification of your early extension. I am pasting reply from HO for my enquiry from
UKBApublicenquiries@UKBA.gsi.gov.uk. Hope this would help.
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Thank you for your enquiry.
Please be advised that maintenance (funds) can include:
cash savings in your account(s) that have been in your account for at least three months before your application;
cash savings in a joint account(s) that have been in your joint account(s) for at least three months before your application;
The balance must show that you have had at least £800 in your account at all times over the three-month period for applications submitted within the United Kingdom, and £2,800 for applications submitted outside the United Kingdom.
Evidence must be in the form of cash funds. If you are able to access your cash ISA account instantly, then this will be acceptable evidence of maintenance (funds).
The personal bank or building society statements should clearly show:
your name;
the account number;
the date of the statement;
the financial institution's name and logo;
transactions covering the three-month period;
that there is enough funds present in the account; and
that there has been enough money present in the account for the past three months.
If your bank statements has all the above information, you do not have to provide a letter from your bank.
For applications made either in the United Kingdom or overseas, all evidence must be dated no more than one month before the application is submitted.
You must have the minimum level of maintenance (funds) available and score 10 points in this section, or your application will be refused.
We recommend that applicants submit an application for an extension around 5 weeks before their visa expires, however, this is not mandatory – applications can be submitted at any time before the current leave expires.
Yours sincerely,
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