I've made reference to going NZ and using old URLs to get to The Lost Corners of BA dot com, but they seem to have more recently created a "classic" URL:

https://www.britishairways.com/trave.../public/en_gb/

in addition the OP should be able to use this direct link

https://www.britishairways.com/trave...b/flightSearch

or

https://www.britishairways.com/trave...b/flightSearch

The last one is deeply down in the former Exec Club area with separate Tokens for NX and non NX so may require you to be logged in a couple of times first, but repeating the above URL typically works eventually.

Those who want to get good at multi-city creation probably need to do the homework and research into (e.g.) ITA Matrix. That's beyond the scope of this thread, but you will find more about this in the Travel Tools forum.