I assume you mean physically take with you on the plane rather than take out by bank transfer for example (for which there is no limit). The concern when taking money with you isn't so much how much you can take out of Turkey, it's the other end - how much you can bring into the UK. I believe from outside the EU the limit is £5,000 - above this it can be considered potential laundered money and can be seized. You can go to court and prove it isn't - but this is very much a "guilty until proven innocent" legal situation. You can also be challenged at your departure airport in Turkey, if they think the money may have been obtained via illegal activity - although this is unlikely (certainly more unlikely than it happening in the UK).
Also bear in mind the security risk of walking around with tens of thousands of pounds in your handbag. The reason people often want to do this is to hide it from the tax man, but bear in mind that if there is a record of the money leaving the UK previously, you should be able to argue that the money is the same as that but returning - and in that case it left the UK "tax paid" and should not be taxable a second time. You must however declare any capital gain (profit) made through the sale and any interest made on the money from the day of it's return for tax purposes.
