First of all, what do you want to do with your computer? Gaming, productivity, or perhaps just web browsing with a lot of tabs opened?
Second, what is the power supply wattage? If it's below 300w then your options are limited.
If the motherboard has a 24 pin connector, the power supply is upgradeable.
An i7 would only give extra threads, but the core count is the same. The biggest upgrade (for gaming) would be a graphics card. For under £100 you can probably find an RX 570/580, even in the 8gb if you are willing to buy used. The problem is that they require quite a lot a power.
There are reasonable PCIe powered graphics cards, like GTX 1050/1050Ti, but they are not worth the money. Anyway, with a weak power supply, none would cut. The only decent option would be the 30w GT 1030, but it's even more overpriced.
I can suggest to get a solid state drive (SSD). They would give a significant boost in OS/program loading, makign the entire system feel faster. You can get a 240gb SSD for about £30.