The classic case for frustration is the burnt down hall case i.e. booking for a dance in a hall frustrated by hall burning down, so contract cannot be performed. I am not too sure it would have direct application to a house purchase, particularly where that property is available.
Afaik banks are continuing to lend but lending criteria has changed. While it might be foreseeable that purchaser would need finance (as happens in many purchases), it is not a requirement of every house purchase - unlike the example above, where existence of dance hall a prerequisite for dance to occur in the hall. Imo, legal certainty would require this doctrine to be narrowly applied.
To address the finance risk the contract is often made 'subject to loan approval'. If purchaser had such approval and signed the contract without this condition, and bank reneged subsequently, then probably need to consider right of action against bank. However, no UK bank has afaik reneged due to insolvency. If closing delayed and approval lapsed or drawdown conditions not capable of being satisfied this is arguably a foreseeable risk of any loan application, where contract unconditional. In addition, there is always a risk that lending criteria or personal circumstances will change at any time up to offer of loan, and this is an assumed risk by the purchaser who signs an unconditional contract for the purchase of a house.
This reminds me of a case in the House or Lords a couple of years ago. A party to a contract claimed force majeure (FM) when commodity prices went so high as to render performance by that party uneconomic. The party was unsuccssful in claiming that this event was unforeseeable and beyond its reasonable control to prevent, and therefore could not rely on the FM clause.