I hate it when the term genuine part is used because it is very ambiguous and creates a misconception that if you don't buy a part from the dealer then somehow that part is not genuine and will void a warranty.
This is a fact with all car manufacturers, they don't make the majority of the parts used in their cars.Literally everything is supplied from another manufacturer, eg Bosch, Siemens, TRW, ATE, NGK, TMD....
Now, when you have some work done at a dealer, you are paying for parts that have been rebranded and have a high markup because dealers operate their parts with a retail mindset, ie for profit. Throw in labour costs which again have a markup element to create profit, this is why costs are high.
You can pickup a throttle body (Bosch) for around £80-£100.
Personally I do as much as I can and do not like using dealers or local garages. However, sometimes I dont have all the tools to do a particular job and if the costs of buying the required tools + parts I need are more or about the same as a dealer price then I might be inclined to use a reputable dealer.
One option you could do is get a break down of the costs in your quote, specifically parts and then ask to supply the parts yourself.