If I have a £25,000 loan, in a balance sheet do I include it in the Cash as well as a liability?
Yes, you are correct: add the 25k both in cash and in liabilities. 

The balance sheet represents what you have (assets) and what you owe (liabilities). So when you get the loan you do have 25,000 in cash under assets, but you also owe it and will need to repay it at some point (which appear as 25,000 in bank loans, under liabilities). As you repay it, the value in liabilities decreases. If you need to learn more there are several free online courses, like this one.