What time of year are you travelling? the only direct Eurostar from London to Avignon that I know of is on Saturdays in the UK school summer holidays from mid July to the end of August. Have no idea whether these stop at Avignon Centre or Avignon TGV but I would assume at the TGV station. If you take the train from the uk to Avignon at other times, you will be on the TGV and stopping at the TGV station just outside Avignon. From there you'll need to get the shuttle bus to Avignon Centre station to get the train to Agde.
This is a lot simpler than it sounds btw, the shuttle costs about 1.5 euro and runs every 20 minutes. It picks up right outside the TGV station and stops just across the road from Avignon centre station.