Yes, the top level domain in your example is .uk and the .ac.uk is a second level domain. the TLD of just .ac on it's own (as already said) means Ascension Island
It's for higher education facilities, but you said .ac in your question, which is not .ac.uk in the same way that .co.uk wouldn't be the same as a .co.ac domain. basically you read domain names backwards and the main defining part is the very last part and then each part before is narrowing it down.