The main problem is that if they needed to change it to pounds again they would get far less sterling. For example, if they had £10,000 which they changed into lira when it was 2.5TL/£, they would have got 25,000 lira.
Today, it isn't 2.5 it's (around) 3.5 - so to buy a pound costs 3.5 lira - so their 25,000 lira only buys £7140 worth of quids today - around 30% less than they changed in the first place.
The other problem with the rate going higher is that people who are selling their property (in pounds) now have a very expensive property in the eyes of Turkish buyers. £100,000 meant it would cost them 250,000 lira at 2.5, but they'll need to find 350,000 lira if they want to buy it today. 100,000 lira more for the same house, and the asking price hasn't changed!!
Hope that helps a bit.