Mathew. The domestic connections are one of the advantages of the BA scheme. The only note of caution is that say you are on London Cairo 4374 miles and you wanted a Glasgow connection 674 miles. That would take you over the Zone 4 threshold to Zone 5 requiring 10,000 additional miles. I think I would look for a reduced domestic flight rather than use 10,000 miles for Gla - Lhr