The overtime rate at LHR ( LGW is different) is not a daily rate but is dependant on your hourly rate multiplied by 1.25 times the credit for the trip. The hourly rate is broadly speaking your annual salary divides by approximately 1050. All this is underpinned by a minimum daily credit of 4.5
All a bit complex but starting overtime rates mean that you get a minimum of appox £280 plus the allowances accrued for the trip.