I don’t really get why you can’t mix and match. I booked a BA Holiday last month paying for WTP into CW with GUF2. I was unable to start from Manchester as there was Avios seats in CE down to Heathrow, but none on the CW flight. But there was “I” class in the CW but none on the Manchester! Even though behind the scenes they told me they still have to ‘convert, the “I” revenue into Avios to process the revenue upgrade I still wasn't allowed to utilise the avios seats on the Domestic! Bizarre really. Ended up having to start at Heathrow as did not want Y down to London. Paid for separate CE tickets. Anyhow with the additional taxes for the domestic it’s hardly worth booking into business anyhow for those legs.