So when it comes to buying a replacement part surely I would want to a buy a genuine component?

Well maybe...and maybe not. The question is: what is a genuine part? What are the options?

PULLED

One way to be confident that your replacement is genuine is to buy one that has been extracted from another iPhone. The term used in the industry is "pulled". A pulled part lived happily in someone else's iPhone, possibly for several years, until something fundamentally failed on that phone.  Then, instead of throwing all the parts away a technician has extracted them made them available to be reused. 

Would you like a used part? In some cases yes. Less CO2 is generated in shipping the pulled part than in manufacturing another. 

IMPROVEMENTS

In some cases a used part works fine. Perhaps your camera needs to be replaced. A pulled or pre-loved camera should work fine although there might be a case for getting a new one. But what if you can find a new camera which fits your phone that Apple produced for a newer iPhone? Maybe it would be better to get the latest technology?

BATTERY

Of course when it comes to  a replacement battery you definitely DON'T want to buy a used one. So is it even possible to get a "genuine"replacement battery? Sometimes you will see a battery advertised as OEM which means "Original Equipment Manufacturer" An OEM battery would imply that the replacement is either supplied by Apple directly or by the same company that supply Apple. That's very difficult to prove and does not mean you are getting the best battery available to you. Battery technology continues to improve

Of course Apple themselves may buy the battery from multiple suppliers. So now do you want to choose the specific manufacturer? Will they even supply a battery to anyone other than Apple? Or do they have an arm that sells an inferior battery to the original but can be marked OEM?

CONCLUSION

What you really want is to be confident that the part you purchase is fit for purpose and has at least the same specification as your original one or maybe even a higher spec. Whats the BEST way to do that.

1. Unless you are buying direct from Apple, DON'T buy a part labelled "Original", "OEM" or "Genuine" (unless of course you want a "pulled" part"). These people are deliberately trying to mislead you and therefore cannot be trusted.
2. If you want to take the greenest approach DO look for a "pulled" component. Why create demand for more manufacture and more shipping unnecessarily.
3. Do buy from a company who offer a "no questions asked" guarantee. The only way that they can afford to offer that service, is if they are confident that you will be happy with the product they sell.
4. If its a camera, a battery or a screen, DO check to see if a better spec than the original is available. 
5. DON'T buy from a generic market place unless you are 100% sure of the provenance

Browse batteries from iPhoneFixUK here.