Turkish Lira (currently about 2.25 to the £) are used in Turkey.
In many of the towns popular with tourists many of the shops and restaurants show their prices in both Lira and Sterling (and some even in Sterling only!) but the conversion rate is not so good. In most tourist locations you can change Sterling into Lira at quite good rates – sometimes better than you can get in the UK. But take cash as many places do not change Travellers’ Cheques, or if they do the rate is poor. If you intend to travel away from the popular spots then I suggest you get your Lira in the UK.
Don’t forget to take a £10 note for your visa fee which is payable upon arrival. I purposely say “a £10 note” because, depending on the mood of the immigration official, they may not accept £5 notes (and they certainly will not accept coins), they may not give you change and (most annoyingly) they may even not accept a £20 note for two people. (They will take the £20 from you, but say this is for one person only and will not give you one of the hundreds of £10 notes they have as change).