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?
UPDATE based on added info in the comments
- It looks like British Airways is offering flights from DEL->LON starting on Jan 7. It's very expensive (US $2000) and the price doesn't come down until Jan 13. London to Munich is bookable starting Jan 10, but the combo seems only available starting at the 13th.
- I didn't find any other Oneworld partner servicing this route at the moment. So rebooking you on a code share is not an option.
- There are plenty of flights on other airlines available. You can see them on Google Flights.
This leaves you with a few different option.
- Call BA and tell them that a Jan 13 return is not acceptable, that you need to be there on Jan 10 at the latest. Tell them that flights are available and they should rebook you on an available one. They will NOT like that since re-booking a non-Oneworld partner is expensive for them, but they are technically required to do so. They will resist and it's hard to force them.
- If that doesn't work: check whether they would rebook you on the 7th to London instead. You can buy a separate LHR->MUC ticket on, say, Lufthansa for less than $200. You may have to pay this out of pocket or you may sweet talk them into paying for this or giving you a voucher. In order for this to work, you MUST be eligible to enter and leave the UK (and be compliant with Covid/Visa rules). Since it's not a single ticket any more you will have to collect your bags in London, go through immigration and customs and recheck to Munich.
I suggest familiarizing yourself with EU passenger rights. See for example https://europa.eu/youreurope/citizens/travel/passenger-rights/air/index_en.htm
Use the term EU261 in the conversation with BA. That's the name of the law and the phone agent will be less likely to pull a fast one on you.