Eating healthily is not necessarily more expensive than eating unhealthily - in fact, it is usually quite the opposite. Focus on changing the balance of your meals. Rather than having a large amount made up of meat or fish (which are expensive), look to have more of the cheaper ingredients like vegetables, pasta, and rice. When you do buy meat look for the cheaper options - pig's liver rather than a rack of lamb. And when you buy fish, consider tinned rather than fresh - tinned oily fish like mackerel are a great option.
Avoid waste. Don't leave food in the fridge or cupboard until it goes off. Do try to get more than one meal from ingredients, for example, a Sunday roast and then Bubble & Squeak on Monday to use up any leftovers.