I do not think the passport and destination is relevant. The facts were clear that I could clear immigration at the destination country with no issue - BA agent had said the block was on the return but still kept me out of the outbound. And that I would have an issue coming back to the UK on the return if they checked my passport at the other end - but no practical issue crossing UK immigration though since I would be using e-gate.
So I had thought the reasonable thing to do for BA was to have let me checked in on the outbound and unloaded me from the return and face whatever issues on the other side - which would have been the case if I had bought a one-way ticket. I was prepared to take the risk as I had saw three potential options of solving the problem (A. Not getting checked on the return - which was very likely. B. Getting an onward flight from the UK to a destination where I am exempt from visa requirements so the UK is "not my final destination" - then cancelling it after getting checked at the airport.)
And to be clear, I am not trying to argue that because I think BA should not stop people from leaving the country without a valid UK visa to return. It is obviously not a very sensible thing to have done (I blame waking up in the middle of a night for the flight....) but given that it had already happen and had a more pressing need to travel and was prepared to take the consequences of being denied the return flight, that BA should have been more helpful than offloading me from the outbound and refusing to do anything about it.
I get that in theory, I may not be given entry into the destination country because I then do not have a "valid return ticket within 90 days of entry". But it was not true in my circumstances as I had a series of flights booked which would have been sufficient proof for immigration at all of the countries but the BA agent simply said that she didn't care what UK immigration says and simply pointed that "the computer says no".
And while it is true that the BRP is required to be brought when leaving the country (which I only found out later), this has never been checked at any UK airport on departure and I have never previously been asked to present this or a UK visa on any previous flight out of the country.
Speaking of eVisa, how does this work then? Do I need a printout of this? I tried showing a scan of the BRP which I had stored on my phone but the same BA agent said that they do not accept pictures because I may have forged it!!!!!