I'd like to travel LHR-BOM on 25 or 26 September one way, in any of the following fare classes:

W/T/E/Y/B/H

I don't need a return flight (but am willing to buy a return sector to throw away if that makes it cheaper, and it seems that one way is more expensive).

Can anyone suggest how best to ticket this? Cathay lets us do things like H+O class, different classes on different sectors. Is there any way to ticket that online for BA or do I need to talk to a TA?

Any help on this would be greatly appreciated - I just need to find the cheapest way to fly in any of the above classes one way LHR-BOM on those dates.

I'm willing to pay a bit more for WTP but it's not really necessary and I'd be willing to live without it. Ideally I'd like an H+O fare where to don't use the return O coupon (so it can booked for any date and also inflexible).

Thanks for any advice.

Edit: I need the flight to be in BA in the specified classes so that I can collect miles for CX Marco Polo. I am aware that IT and 9W are probably cheaper.