You have a few options with leftover travel cash:
1) Store it away for a future holiday
2) Take it to an exchange to convert it back into GBP
If you choose the latter option, you may be hit by 'buy-back rates'. These are the rates offered by exchanges to take back unused Euros, Lira and other foreign currencies. Often then are much worse than what you originally bought the currency for.
That's why we always recommend using a travel card. You can convert back and forth between pounds and foreign currencies at the market exchange rate, meaning you don't get punished for not spending all your travel cash!