Hi I am travelling from Harrow on the Hill to Grays in essex by train, and
need to be in Grays by 12.30.

I will be making a return journey as well

I have a PAYG oyster card,

whats the best way of buying a ticket for the BR part of the journey? I dont
want to double pay, as I feel my oyster would become maxed out as a travel
card, so I need a zone 6 to Grays extension, unless Oyster is availible at
Grays.

Can I buy this at a LU station?

Thanks for your help.