I think people who warn against putting money into lira accounts are thinking about situations where someone has saved a nest egg - say £100,000 - and is dazzled by the 10% interest rates sometimes available on 30 day notice accounts and similar. That is, we are talking about people investing large sums of money into a Turkish bank. For this you need to make a judgement about return versus risk. Why do Turkish banks offer higher rates on lira accounts than they do on sterling accounts? Because they are perceived, by Turks, to be riskier. Turks with long memories can remember the lira suddenly falling through the floor, and on more than one occasion. They prefer gold or dollars.
However, you are not talking about that. You are dealing with £500-odd a month, much of which will probably be spent that month. So there isn't a lot at stake and it isn't over a long period of time. There is less to worry about.
P.S. Using a Nationwide Flex Account card in a Turkish ATM will cost you £3 per £100-worth.