Any Londoners? Quickest/easiest way from Hemel Hempstead into central London please

[MadameGazelle]

We're going to the Harry Potter studio next month, travelling by car and staying at a hotel in Hemel Hempstead. We want to combine this trip with a day in London and wondered what would be the easiest way to travel into London on public transport for the day. I've looked at an off peak, one day travel card for zones 1-6 (bus then the tube) but but can't find out if the bus from HH would be covered by the travel card?

If anyone can suggest an easier or better way that would be great. We're a family of 4 - 2 adults and 2 kids aged 14 and 13. TIA

If using contactless you need individual cards for each person. Your teens would have to have their own (can under 18s have contactless?). A lot of people seem to not be aware of that.

@MadameGazelle Parking shouldn't be too bad at the weekend.

Just throwing another suggestion out there- drive into London! It's quick, easy, no congestion charge on Saturdays. Park in a Q-park i.e in Chinatown and it'll be the same price as 4 travel cards.