Absolutely you can mix different octanes of petrol (gasoline) in your car, it won't hurt your car and you may get better performance with a more expensive fuel. Premium fuels are generally better quality, have more energy per unit as well as detergents so they definitely won't hurt (I'm not advocating premium fuel by the way, I think it's a ripoff most of the time). Most cars these days have octane sensors and adjust automatically to use whatever they are given.
However, you may not actually save money. Assuming UK money premium fuel is generally £0.20 more expensive per litre, it would only take 14 litres to make up the price difference, and then you are spending more money.