How much are we talking about here?
Don't carry large amounts of cash - it is not safe, and also UK customs impose limits (10,000 Euro) on cash physically taken out of the country (to non-EU countries), unless you complete a declaration form. In fact, there are powers in place that allow seizure of any amount over £1000 if it is suspected that the money came from, or is intended for use in illegal activity. That doesn't mean they have to prove (initially) any unlawful activity, they can seize based on suspicion alone - then your plans foryour time over here are well and truly scuppered.
There really is no need to carry cash. Ask your bank to transfer it - 2 to 5 days is about the norm.
I assume you are transferring the money to your own (Turkish) account. If you are transferring it to any other - a developer or agent, be entirely sure that you have got exactly what you believe you are paying for.