Depending upon your carrier and duration of trip, there are many, many options. For a short trip and to make it more special (especially after a trip down the M4 from yur city, may luvver
) you might want to take one of the biz parking sites, like Pink Elephant.
The Flightpath at the end of 27R is still a good one BUT you should pre-book to get a better deal. One route is through the BAA website but folks like aph.com also take bookings. I have used them before and it worked.
As always, each web site offers their own offers and you need to shop around. BAA are not always going to tell you about every possibility, of course, as they want their cut.
Lastly, for a short trip I do NOT advise off-site parking. The extra time taken to find the place and get to/from the terminal is just not worth the cost saving. I once found myself saving about ten pounds but spending more than an extra 90 minutes (in total) and it was a painful 90 minutes.
Lastly, the old Reading/LHR coach still operates. However, this is really only good if you are going from T3 as it goes (naturally) to the central station and if on big airways, you have to go down to take the train over to 4 and that crosses the cost benefit analysis path.