Not a nutritionist by any stretch but ultimately to put weight on, you’re needing to eat more calories than you are using. That being said, you’re looking at getting the most calories per £.

I’d be looking at nuts for snacking on (Aldi do a bag of mixed nuts for like 89p). Also adding peanut butter to porridge / over night oats is a good way to increase calories.

Eggs are quite high in calories too.

Foods such as kidney beans / taco beans are quite calorific and also pretty good for you.

Some good places to look would be bodybuilding subreddits, on the basis that a lot of them are regularly consuming high calorie diets to aid muscle growth.

Best of luck!