This is tricky and Covid makes it even more so.
- Typically a refund is owed by whoever took your payment. Take a look on your credit card: If the payee is Opodo, you should primarily talk to them. If it’s BA, they are your primary target. This really depends on the contract between BA and Opodo and type of booking, so you should check.
- Partial refunds are difficult since the legs often don’t have individual prices. They are bought as a “package” and often a round trip is cheaper than a one way and it’s almost always much cheaper than two one-ways.
- In order to determine a refund, one would have to price the un-flown leg based on a round trip purchase & fare rules of the original ticket. I’m pretty sure BA could do that if they wanted, Opodo probably can’t.
- Even if you get a refund, you are still stuck in Delhi. The refund is unlikely to cover the price of a new one-way ticket
- The “normal” procedure here would be that the airline offers you an alternative return flight at no extra cost to you. The sold you transportation to Delhi and back and that’s what they are required to provide. That can backfire as well: they may offer you alternative flights, but if you are not allowed to enter the UK you can’t take them and that’s your problem and no theirs.
Your best of course action depends a bit on your specific situation. Do you want/need to go back to London? How soon, how urgent? Can you get into the UK, i.e. what’s your citizenship/visa and how does it relate to Covid related restrictions?