I was going to post on this but reading PAXboy's comments .... we've been in pretty much the same situation and come to the same views.
Large company and 'common sense' travel policy just don't go together. I got back from Sydney once to LHR and due to the stupid company travel agency had no onward ticket to my final destination. A BA o/w would have been £450, so I hired a car, booked an easyJet from LTN, went to see my sister for lunch on the way, and headed back. Total cost ..... £75. Put my expenses in and was called into my director's office for 'contravening company travel policy' by booking an LCC. I asked him what the company policy was for and he fell right into it : "To save money ....!" he said. So I said : "Think about what you've just done and said ...." and walked out.
.... and another thing. I also worked as a consultant and a client would often consider that your travel and hotel expenses were a benefit to you and would negotiate to take them out of your fees!
Another time they sent to me to WTM in London. I stayed with family in North London and took a minicab to and/from the fair every day (I can't use the Tube), cost £25/day. My colleagues stayed in a Hilton about a mile from the fair and paid well over £100/day plus taxis because they were too lazy to walk. My expenses were rejected because they could not pay transportation expenses which exceeded 35% of hotel expenses, therefore hotel expenses '0', no transport expenses allowed. Wucking Fankers!!!