Gatwick, Stansted and Southend all have rail connections to the terminals. Luton has a station but it is a short bus ride away from the terminal. Depending on where exactly you are starting from in London the train service to Gatwick can be easier than the tube is to Heathrow, and as long as you avoid the Heathrow Express trains it is relatively inexpensive.

Having said all of that it would have to be a significant saving to steer me away from Eurostar if you want to travel from city centre to city centre with luggage.