When I come to book an Amex 2-4-1 I always mark my departure as my regional airport. I then check the availability using the Amex 2-4-1 and do exactly the same were I to use BA miles only as is some cases I might prefer to hold onto my 2-4-1 for a later date. As I stated previously it is very often the case that the connecting flights to LHR available using BA miles only are not available when I check using an Amex voucher redemption. I checked flights to the USA yesterday and the same happened and in fact it was an earlier connecting flight into LHR that had disappeared leaving only a connecting flight with a minimum connecting time available. I had hoped to make good use of the CCR between flights but the connecting time available using the Amex voucher only gave me just over 1.5 hours at LHR. When I went back in and re-checked using BA miles only the original domestic flights to LHR offered were still avaiIable. I have to go out now but I will try and give an example tomorrow if someone is prepared to look at this.
All the connections are BA to LHR. No codeshare involved.