The ONLY site that you should ever use for rail times and bookings is National Rail.
http://www.nationalrail.co.uk/
That site directs you to purchasing options which don't add any charges.
For the route you refer to, your request to purchase tickets will be routed to the First Great Western website.
However, as you've suggested, any staffed station throughout the UK can sell you tickets for almost any route. (There might be a few, very limited, routing exceptions, relating to 'special offer' tickets from specific rail operators, but they wouldn't apply in your situation).
Chris