Hi Rach
Quite an interesting thread on the subject here probobly worth absorbing:
https://www.audi-sport.net/xf/threads/s3-first-oil-change-cost.306947/
That said if your local dealer in Poole seems expensive phone a few futher away to get some quotes it can vary and that said your local dealer should price match. There is also nothing stopping you taking it to a Audi specialist garage this should also be a cheaper option. It is worth noting that should you go down this route not having the proper service stamps from an Audi dealer can impact resale value later on. Hope this helps, all the best!
Audi Specialist in Poole
https://www.google.com/search?safe=strict&client=firefox-b-ab&ei=X8xQXI6RCdLexgP4sr7YBA&q=audi+specialists+poole&oq=audi+specialists+poole&gs_l=psy-ab.3..0.12570.17614..17876...0.0..0.132.1374.1j11......0....1..gws-wiz.......0i71j0i13j0i7i30j0i8i7i30j0i13i30.i0xUNF_vtao