Rail/tube questions

For all transit in London (rail, tube, bus), get an Oystercard at the first Tube station you go to. On the card, you can either store (a) cash, for pay as you go; or (b) an all-you-can-ride Travelcard (pass). Which makes more sense depends on how long you plan to stay in London and how many rides you expect to take. I usually find pay as you go to be the best deal.

Between London and Heathrow, I personally take the Tube. Backpacks or one piece of luggage per person is no problem on that line, just put your bags at the end of the car. If more luggage than that, you'll likely prefer the Heathrow Connect rail line to Paddington.

See the Transport for London website for all maps and ticketing information.

Doubt you will need to pre book your train from Oxford to London but you should research cost and times.

It is a train ride to Harry Potter Studios and you should pre book your admission tickets immediately. VERY popular and you are traveling in the height of tourist season.

https://www.wbstudiotour.co.uk/getting-here

NOT worth getting a rail pass.

Are you wishing to spend an hour on the tube for the cheaper ride to Heathrow via the Piccadilly line or willing to cough up some change for either the Heathrow Connect or Heathrow Express.

The latter two are more expensive although you can pre book and save money.

The Express is a quick 15 minute ride from Paddington Station.

Amazing thing Google.

You don't need a rail ticket for either HP studios or Heathrow. You can use your Oyster card if you take the tube to Heathrow. It also covers the train from Euston station to Watford Jct. It You will have to pay cash, however for the bus that shuttles you to the studio from the train station.

I would seriously consider buying the Oxford-London rail tickets ahead of time and would certainly do so if I was certain about my travel date and time. The fares seem to range from £12 to £32.60 per person. The cheapest fares are "Advance" fares into Marylebone Station. If you must go direct to Paddington, the best you can get seems to be an advance fare of £26 (less on Sundays).

https://tfl.gov.uk/
Official site with interactive trip planner.
You can compare the info on Google Maps, too, which is getting better with mass transit. Google's link varies depending on where you live.

You can use either a rail ticket for the short journey from Euston to Watford Junction or tap in and tap out with an Oyster. Be aware that if you are aware of the different zone prices that Euston is in Zone 1 and Watford Junction is in its own special Zone 10.

Heathrow, for example is only in Zone 6.

Both Emma and Claudia have the right thoughts but from different angles.

If you are on the north side of Hyde Park or near Paddington the Express makes a lot of sense. If you aren't, changing from Tube to train after getting around to Paddington is a pain and takes a lot of the time you would have saved on the faster train. If you are near the Piccadilly Line just using that works well and is nice and cheap.

So you can see that in all of these cases knowing where you will be in London makes a lot of difference.

No rail pass.

The Chiltern route to Marylebone does not always have the cheapest Advances to London as GWR often uncuts with promotions to Paddington. The latter is somewhat better directly connected into the tube network. The cheapest fare can be as low as £5.