Yes, that will be £13.30 using Liz line and Thameslink. I'd choose that as I prefer the spaciousness of the Liz line trains. Board at the rear at Heathrow to be nearest the lifts / escalators at Farringdon for the transfer to Thameslink. You'll might see confusing signs at the interchange saying you need to tap the oyster reader - that only applies if you had done the first part of the journey with a paper ticket. As you'll be using oyster/contactless for the whole journey, you can ignore that, you only need to tap again at the exit at St Pancras