Your coffee will be fine so long as it is declared. The oatcakes/biscuits should be fine as long as they are in unopened, commercial packaging and are fully cooked.

As noted above and you seem to have picked up on, simply declare that you are carrying food on your incoming passenger card. The Customs officer at the primary immigration line will likely ask what you are carrying. If the answers are fine you will get the right markings on your passenger card and be sent either straight through the customs and quarantine line or simply have your bags x-rayed.