Whatever your preference is. Turkish Lira TRY to GBP is outstanding at the moment.
On Wise.com card you are getting 26.25 TRY to £1 GBP so I’ve loaded 2500 TRY on to that and changed £100 at Tesco and got 2300 TRY. I like to have cash in my pocket too.
You’ll get a good rate on your bank card anyway (just make sure you ask to pay in TRY not GBP) but you’ll potentially get bank charges too, though shouldn’t be too much.
Some have mentioned paying in Euros, which are widely accepted, but you won’t get as much value out of them as you do the Turkish Lira.