Although you can still purchase top-up/pay-as-you-go Oyster (transport/travel) Cards (like a Myki card) in London, there’s now a better & easier system, without the hassle of finding the correct change or calculating the fare cost.
At the turnstile gates, you can just tap (on) your credit card or debit card onto the scanner. The light will turn green & the gate will open for you, so you can enter to access your train platform. When you get off at your stop, again tap (off) your credit card or debit card at the turnstile gates to exit the platform & station. Your credit card or debit card will be charged the exact fare for that journey.
When booking hotels in London, book a hotel room that is walking distance to an underground/tube station that has a wheelchair symbol within its underground map station icon circle. This indicates that this tube station has lifts, and you don’t have to lift your heavy luggage up & down copious amounts of steep stairs. We learnt this lesson the hard way.
And be prepared for wide gaps between the train’s doorway and the platform itself, not to mention a difference in height between the two. It’s rare that you can just step on easily between them.