Check with you debit cards if they levy a transaction fee for every transaction.
Taking cash out with a UK debit card in Germany costs you around 3.50 GBP. So if you can live with that, that could be the easiest route.
Obviously, when you buy currency, you will also have to exchange it when you get back – whic costs and is a PITA. I have 160 GBP worth of roubles are the moment – I will have to wait until I am at Heathrow!
Do NOT cchange at airports: the exchnage rates are the louiest in the world. They were offering around 1 GBP : 1 Euro this morning ….