The train station in Borehamwood is called Elstree (strictly Elstree & Borehamwood) but is pretty much in the centre of Borehamwood, whereas Elstree town is 2 miles from the station. Don't ask how this happened!
It is on the "Thameslink" line that comes through London Bridge and Kings Cross in the centre of London, and goes north to Luton. If you are coming from Heathrow take the Piccadilly Line Underground to Kings Cross, and change there following the signs for the Thameslink station (it is a separate station 200m from the main Kings Cross station, but there are passages under the street to walk through).
Trains from Kings Cross to Elstree every 15 minutes. Journey time from Kings Cross is 22 minutes.
If you are going to the TV studios in Borehamwood they are walking distance from the station, on both sides of the main road. Otherwise there are local cabs at the station.
Those hotels are pretty dreary. The Holiday Inn South Mimms is admittedly cheap but is in a motorway service station on the M25 London Ring motorway, and you have no car. Why not stay in Central london and go out on the train each day?