I queried Avanti over charging the new fares too early, but I got a predictably useless response:
Ah, the typical attempt to shoehorn in an unhelpful metaphor from a completely different industry... Surprised they didn't try and quote one with BA and Virgin - that's the usual refrain (even though of course the rail industry works under completely different principles)!
If they want to go down that road, perhaps you should reply that it's perfectly normal for retailers to encourage short-term sales growth by allowing people to lock in current prices when a price rise is announced - even if the goods or services are only provided after the price rise takes effect.
Ultimately, the issue you have is that there's no question of them misleading you or otherwise being in breach of contract or consumer law. They quoted you a price, you agreed to it and a contract was formed on those terms. So you are essentially reliant on their goodwill to get your partial refund.
There's nothing stopping you from ultimately taking it to the Ombudsman though, if you feel thus inclined.