Hi Adam,
There is only one fare set between Kentish Town and Luton as that comes under the Project Oval setting. The fares are cheaper than from St Pancras, so as long as you adhere to any interchange times you should get that fare via St Pancras as well.
With St Albans (or Luton Airport Parkway) it will depend on whether the gates for the EMR platforms trigger the expensive route or if it’s just the Underground barriers at Kings Cross St Pancras. The Thameslink fast trains are not much slower than EMR and it’s a longer walk at St Pancras, so I’d probably stick to them for certainty.