Best pass and trains for Cotswolds

Hello! We are arriving at Heathrow in early September. We are going to Windsor for a few days, then on to the Cotswolds. We do not want to rent a car. What is the best way to visit the area by train?
What lines should we use and would a pass save us money for two weeks? We are a bit confused about the pass. We are senior citizens and cannot walk long distances. We must depend on public transport. Thank you!
This is me with my 11x Great Grandfather, Sir Edward Littleton in Penkridge, UK. 2017 our 50th anniversary
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