Are "EC cards" from banks in Germany valid in UK ?

EC cards are the euro check cards(similar to debit cards) that a bank issues and which can be used in all euro-countries.

not in UK

Depending on the bank, I have a Postbank account and my EC Card is good anywhere in the world as long as the foreign bank machine accepts Maestro. I've used mine in Canada, UK, France, Spain, and Austria

The answer from Germanda is correct. I have an EC card that uses the Maestro ATM network. So just make sure the bank ATM that you use has that symbol on it. Or, look at the back of your EC card for the ATM systems shown, and then look for the same ones on the ATM's in the UK. That is the 'Tried&True' method I have used in my travels. I often hear that EC cards do not work in the UK, mainly because they do not use the Euro. But, EC cards do work - especially at the ATM by Harrod's.

I never had problems to get money at a bank cashier in London with my EC card.

If your card has the Maestro symbol on it. It should work at merchants and automatic teller machines that take Maestro. Check with your bank it may need to be activated for international transactions.