Rio De Janeiro Avios pricing

Hello! Excuse the lengthy post, I have a 2-4-1 Avios booking next year for a trip from Manchester to South America. Initially booked the outbound online with CV to Santiago using Reward Flight Saver. Booked the return from Rio to Heathrow online when it went on sale benefiting from the low Brazilian taxes of just £18 for 2 in Club!

I called BA a couple of days after this to refund 50% Avios and to add the domestic leg back up north to Manchester and was quoted an additional charge of £18. I’ve since received an email from BA asking me to contact them and it turns out that the additional charge was quoted incorrectly and they now need authorisation for an extra £200!

When booking GIG-LHR-MAN in J online the price charged is 200,000 Avios + £35.44. I challenged BA on this and they’ve advised that BA.com is incorrect and this is an error fare which they’re willing to honour for GIG-LHR but not LHR-MAN which is resulting in the £200 additional charge. Is this correct?

It seems odd to me that the website pricing would be incorrect as I have been monitoring this for the last few weeks since settling on South America, and it seems like a lot of revenue for BA to be throwing away honouring error fares for a known issue.

They’ve offered the suggestion of cancelling the booking and then seeing if the seats get released and then booking again online, I don’t really want to risk losing the seats we have, also with cancellation charges it wouldn’t be hugely worthwhile vice just paying the £200 requested.