Originally Posted by
windowontheAside
It's debatable if that would count as a website bug so you could get service fee waived, but a sympathetic agent might be able to do so.
Originally Posted by
Andriyko
Must be a ba.com bug. I checked the rules of various fares for this route and there are no restrictions with regard to combinations. You should be able to purchase such a ticket over the phone.
On a quick look, this seems to be a bug.
This itinerary will price in ITA (N class outbound 13 May, I class inbound 18 May): £736.50 + TFC = £1,128.26. Fare bases NLNCGB and IAP9UK.
There are some I class fares that have very restrictive combinability rules, but this one is obviously not one of them.