Apple

Apple's official policy is you can't access that money as cash "except as required by law".

In other words, if UK law says you are entitled to the money, then Apple would know that, and you simply need to contact Apple and ask them to give the money to you, as cash (or a bank transfer).

They don't like doing it, they want you to spend the money with them, but in many countries they are required to by law. Because it legally is your money - Apple classifies it as being spent when you use it to buy something, not when you add to your balance.

Depending how the money was deposited in your account, you might not get all of it back. For example target often sells $100 Gift Vouchers for $80. Try to get them to give you $100 cash in that situation, and there's a good chance Apple will only give you $80 (Target didn't make a loss of $20 on that gift card, and neither will Apple). But it doesn't sound like that will affect you.

PS: Transmit is awesome. Been a user for decades. The Mac App Store version is more expensive (subscription pricing with a lot of the money going to Apple) and has less features (some important features are not allowed in App Store apps) - so yeah, don't buy it from the App Store.