The OP needs to establish 1) whether he/she wants to save money, or 2) save time. If it's the former, then back to backs are the way to go - and, of course, no checked bags. If it's the latter, pay to fly from a London airport.

In any event, very often deeply discounted biz fares, taxes included, are lower from LHR than they are from elsewhere as it's a much more competitive market. The day's of really cheap flights from DUB or Scandinavia seem to be gone.