Okay, anyone who imports one on your behalf is gonna take some £££ for the trouble.
Your question of why not deal with Motordyne direct, is a fair point, some people do. Some people use american companies who stock other manufacturing brands and source all their parts in one place.
I've purchased used on my350, from outlets in America as well as using UK based suppliers like Tarmac.
The hassle of self importing isn't that bad. It's simple yet effective. The point of having somebody in the UK obtain the parts for you, means that they are taking the responsibility of ensuring you get the part.
If you have any hassles obtaining parts then you must deal with it. Although I've never had an issue, and I know Adrian (Octet) has brought in I'd say in excess of £25k of parts if not more into the UK, not just for himself. This saves on delivery costs with bulking the items together.