there is a tesco at slough station that the number 7 and number 8 buses go to from LHR. no oyster available on the 7 or 8 as its not one of the london buses (maybe first berkshire?)
there is also a sainsburys on the way into slough which I suspect these buses go past.
Also the 81 along the bath road goes past these supermarkets. Oyster is available on the 81. the 81 does not go past any heathrow terminals so would involve a change if you choose to use that bus.
This would be my choice too. I’ve stayed along Bath Road and taken bus 81 to either the big Sainsbury’s on the way into Slough (on the right hand side of the street heading from LHR to Slough) and an even bigger Tesco opposite the bus station terminus - last stop on the route.
I have done the trip a number of times. Easy on the bus, and you can use contactless debit or credit as well as Oyster card (£1.50 flat fare) but cash isn’t accepted.
I would allow 3 hours for the round trip including shopping.
enjoy!!