There are no provisions to ‘extend‘ a visa from outside the U.K.
Extending a visitor visa from inside the U.K. is disproportionately expensive.
Applying for a new visitor visa from outside the U.K. should be okay. There’s no harm in asking the ECO to waive or refund the visa fee because of holder’s inability to use the previous visa due to travel restrictions during the pandemic. But they may not have the discretionary powers to comply with the request.
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