You say you love coffee but then mention Greggs and McDonalds as examples of good coffee which is like someone saying they are a foodie and mentioning, well, Greggs and McDonalds.
If you want something similar to them then they are push button machines, not barista made, which means you are looking for a bean to cup machine which you can easily get at home, but can be pricy. You don't mention what type of drink you like. Is it a milk based espresso drink like a Latte or something else like an Americano?
If you want the authentic Americano from Greggs experience at home then you should be able to do that with a Pod machine. Pod coffee only contains ground coffee so there shouldn't be anything in there different to any other brewing method. If you are just after a black/white coffee and it doesn't have to be anything fancy then a filter will give you what you want and doesn't need any complicated setup if you use a machine, and you can get machines for as little as £25. Manual ones are even less but it's a more manual process. French Press, Aeropress or moka pot will all make good coffee.
The biggest difference you can make with the quality of your coffee at home is to learn a bit about coffee, buy good quality, freshly roasted beans from a roaster (not a supermarket), grind yourself (but a good grinder is expensive and you are better off buying pre-ground (from a roaster) than using a rubbish grinder) to the correct grind size, weigh and time your brew so you are getting the most out of the better quality product.
Coffee bags seem like a ridiculous concept. They are essentially an expensive way of reproducing an immersion method like french press or aero press but with rubbish coffee.
