I previously (only last month) logged in with booking.com/britishairways and got the same price quotes on accommodation, as if I hadn't gone via the portal. (with recognition that I'd get xx avios credited to my account - equivalent to 8 per £ spent.)
Now I can only log in via the portal, but the prices shown are inflated by 8% compared to booking on the mobile app, so if you want to buy Avios at 1p each, it's ok- otherwise, don't bother!