My wifes car is due for service and unshow if I should use the main dealer or a local garage theres a £100 difference in the price and my local garage will do more. Just unshow as of warrantys

Needs to be a vat registered garage and use parts and oils that meet the manufacturers specification , they must also be listed on the invoice just in case of any futre problems . Some brands rely on dealer history for a better second hand / resale value .

Believe they have to be genuine parts?

For £100 it isn’t worth the risk.

The risk? That after the warranty expires you get a problem that if it was within warranty the manufacturer/dealer would fix FOC (or at least contribute). But because you are out, requires them to show ‘goodwill’, and if you’ve serviced it elsewhere that goodwill is less likely.

And if you can afford a new car, then budget for dealer services.

It does depend a bit on the car and it’s value but as above any proper garage can do the service with parts and oils that match the manufacturers specifications. In reality this means using genuine parts as trying to prove another one you’d used was the same quality etc in case of the need for a warranty claim would be a huge effort.
It will help residual value being done by main dealer so that would be my choice.